John Tlumacki/The Boston Globe/Getty Images

By MICHAEL FALCONE ( @michaelpfalcone )

NOTABLES

• AUTHORITIES RACE FOR CLUES: Hundreds of U.S. law enforcement agents are racing to gather and analyze every shred of evidence from yesterday's deadly Boston Marathon bombing for a hint of the attacker, but the morning after three were killed and nearly 150 more injured in twin explosions, no one has been taken into custody, reports ABC's BRIAN ROSS , RHONDA SCHWARTZ and NED BERKOWITZ. Overnight a tip about possible explosives led federal agents to search an apartment on the fifth floor of a building in the Boston suburb of Revere. Agents later told residents there was nothing to worry about, but the quick and overwhelming law enforcement response underscored the urgency of the FBI's effort to track and stop the people responsible. FBI agents also went to a local hospital to question a 20-year-old Saudi college student who was injured in the blast, but authorities stressed that he is not considered a suspect. Experts said the most important clues could come from several videos from the race that caught the blasts in real time as well as the explosions themselves. ABC's JIM AVILA reviews the anatomy of yesterday's bombs: crude, unsophisticated, but deadly. WATCH: http://abcn.ws/118WoMp

• INTEL COMMUNITY 'PERPLEXED': ABC's Senior Justice Department Correspondent PIERRE THOMAS reports that the law enforcement intelligence community are perplexed that there's no chatter or claims of responsibility. Typically, according to one senior official, terror organizations like to claim responsibility while the event is unfolding. Piecing together the bomb remnants will be key because the authorities have a vast catalogue of past bombing incidents. The description of these devises will be compared to the designs of bombs in the database. The database will churn out the types of groups known to make those specific types of devices. But finding the Marathon Bomber could take months, former counter-terrorism official Richard Clarke noted on "Good Morning America" today. WATCH: http://abcn.ws/ZuBg07

• AMONG THE VICTIMS: A boy who died after waiting for his dad to finish the Boston Marathon was a typical 8-year-old who loved to ride his bike and play baseball, according to a neighbor. ABC's ALYSSA NEWCOMB and ANTHONY CASTELLANO report that Martin Richard died from the explosion that injured his mother and sister as they all waited for his father to finish the race, according to ABC News' Boston affiliate WCVB-TV. A single candle was placed in front of his home overnight in Dorchester, Mass. http://abcn.ws/11lKsGV

• THE HEROES: Dr. Vivek Shah was among those who provided on-site assistance to victims yesterday. He had just crossed the 26.2-mile finish line at the marathon Monday when he was forced to put his medical skills to work. "I've never obviously been in combat, but people I've trained with have been and this is as close as I can imagine it would be," Shah told ABC News Radio late Monday night. "Just, basically piles of victims. Everything I saw was a traumatic amputation, basically." Shah, an orthopedic surgeon at New England Baptist Hospital in Roxbury Crossing, Mass., said he saw injuries along the sidewalks on Boylston Street for which no amount of training could prepare him. "In all my medical training, I have not seen things that I saw [Monday]. Everything was traumatic," Shah said. http://abcn.ws/11lKsGV On "Good Morning America" today, Dr. Alan Panter, an emergency-room doctor, discussed what he saw and how he tried to save lives. WATCH: http://abcn.ws/ZXMBVw

• AT THE WHITE HOUSE: President Obama is being regularly briefed on the response and investigation into the Boston attack. This morning, he will receive a briefing from his Assistant for Homeland Security and Counterterrorism Lisa Monaco, FBI Director Robert Mueller and other senior members of his team. A White House official tells ABC News: "The President made clear that he expects to be kept up to date on any developments and directed his team to make sure that all federal resources that can support these efforts, including the investigation being led by the FBI, be made available."

THE ROUNDTABLE

ABC's RICK KLEIN: The toll on Boston, and on the country, will be deeper than the number of those killed and wounded. Someone or more than one person blasted through a city and a nation's sense of security. By striking an iconic event, on a symbolic day, at a time when Americans and those from around the world were coming together, an unthinkable tragedy became more than that. It was the kind of attack authorities have worried over for so long, and now that it's happened, there will be a before and an after. Politics recedes for a few days, maybe less. It will be back. It will take far longer to recover what else was lost in Boston.

ABC's JEFF ZELENY: The spotlight is not shining as brightly on the gun control debate today, given the attack in Boston. But an intense scramble is underway to keep the compromise alive on background checks. So far, supporters don't have the 60 votes they need and are trying to make the bill more acceptable to a handful of Republicans and Democrats from gun-friendly states. It could be a tall order, judging from our talks with senators. A key goal of the day: Trying to keep Senator John McCain on board, which supporters of Manchin-Toomey believe will provide "cover" for others to join.

ABC's Z. BYRON WOLF: The political fights on which Washington was so narrowly focused less than 24 hours ago seem distant and fuzzy this morning. We will return or attention to background checks and a pathway to citizenship at some point, but the tragic Boston Marathon bombing is the kind of event that will definitively change the subject and hold the national attention regardless of who is responsible.

FUSION's JORDAN FABIAN: The fact that the Senate's "Gang of Eight" (finally) introduced its immigration is an achievement in itself. But now the real tough task begins. The Senate group made a painstaking effort to ensure that undocumented immigrants will have to earn their way to citizenship, and not just have it handed to them. But Republicans reform-backers, such as Marco Rubio and John McCain, now face the herculean task of selling cynics in their party that their plan isn't simply "amnesty" that forgives law-breakers. Democrats could encounter a lesser, but not insignificant, challenge from immigrant rights activists. They may object to cut backs in family-based immigration and the billions of dollars spent on new border security measures at a time when administration officials say the southern border is safer than ever. It took a political high-wire act to keep the "Gang" together long enough to produce a bill. But the real challenge for the group lies in cobbling together a strong majority for a bill that won't please everyone, to put it mildly. http://abcn.ws/13fypeo

BUZZ

'GANG OF 8' TO INTRODUCE IMMIGRATION BILL. The Senate "Gang of 8? that has been leading the charge on immigration reform has signed off on a comprehensive bill that will be introduced today, reports ABC's JIM AVILA. The group of bipartisan senators led by Arizona's John McCain, South Carolina's Lindsey Graham, New York's Charles Schumer and Florida's Marco Rubio came to terms on one of the most contentious elements of immigration reform: border security. The bill requires "persistent surveillance" of America's southern border and a border security effectiveness rate of 90 percent before allowing undocumented immigrants who arrived in the United States before Dec. 31, 2011 to apply for legal status, setting them off on a 10-year path to citizenship. The bill appropriates $4.5 billion to add 3,500 border patrol agents, install surveillance systems, add drones and build more double-layered fencing. The "pathway to citizenship," which would allow many of the 11 million undocumented immigrants eventually to become citizens, requires immigrants to be free of felony convictions and pay a $500 penalty fee, plus back taxes. Those who qualify would gain legal status immediately, but go to the end of the line for citizenship. After a 10-year wait, they would then be granted green cards, followed by full citizenship. http://abcn.ws/118PEeL

HAPPENING TODAY: A Senate delegation led by Sen. Schumer, D-N.Y., is expected to brief the president later today at the White House, according to sources, but a news conference previously scheduled to announce the reform bill on Tuesday has been cancelled following the Boston bombing.

MEANWHILE, IN THE HOUSE… Rep. Xavier Becerra, a key player in the House's own select group of lawmakers working to fix the country's immigration system, says lawmakers in the lower chamber are finalizing their proposal as well, ABC's JOHN PARKINSON notes. "If the Senate does announce tomorrow, that'll be great because it'll give everyone a chance to see what a bipartisan immigration reform can look like," Becerra, D-Calif., said during a news conference outside the Capitol today. "I don't think the House will be too far behind. I think, in fact, both the House negotiators and the Senate negotiators are trying to iron out for the most part smaller issues now." Becerra, 55, one of four House Democrats at the center of the negotiations, would not specify how soon the House Gang's plan could be finalized, but he predicted that the plan will be similar to the Senate group's proposal. "Progress is being made," he said. "You're not hearing a lot of talk about amnesty and self-deportation any more. You're hearing talk about how we get this done. http://abcn.ws/16YUwIv

POLL: TWO-THIRDS OF AMERICANS BACK A PATH FOR IMMIGRANTS. ABC News Pollster GARY LANGER reports: Americans by nearly 2-1, 63-33 percent, support a program giving undocumented immigrants a path to legal status, according to an ABC News-Washington Post poll out today. That reflects a sharp change in the past five years, from essentially an even split on the issue, 49-46 percent, in late 2007. (Another question, posing a "path to citizenship," found 57 percent support late last month.) The results reflect continued challenges for Republican leaders seeking compromise on the issue. Current support for a process to achieve legal status falls to 47 percent among Republicans, vs. 64 percent of independents and 75 percent of Democrats. And it's gained an insignificant 5 percentage points among Republicans since 2007, compared with advances of 19 and 16 points among independents and Democrats, respectively. Other elements of immigration reform also hold majority support - 60 percent for more visas for highly skilled workers and 56 percent for a low-skilled guest worker program. On the enforcement side, 67 percent are for more federal spending on border control and a vast 83 percent favor requiring that all businesses check potential employees' immigration status. http://abcn.ws/Zxyp5J

GUN VOTE PUSHED BACK. The Senate vote on the Manchin-Toomey background check provision of the gun control legislation will not be held today, Senate aides tell ABC's JEFF ZELENY, and possibly not until Thursday or next week. The delay is not related to the Boston attack. It was being discussed earlier today, hours before the incident. Senator Joe Manchin, when asked if his amendment is in trouble, said: "Trouble? Nothings ever in trouble. It's not over till it's over. We're getting there." It's clear supporters do not yet have the 60 votes that are needed. The outcome of the bill is in question now, but a handful of senators are undecided and a few others could change their minds. "It's razor thin," Senator Chris Murphy, Democrat of Connecticut, tells ABC News, adding that he is optimistic the legislation still has momentum. Gabby Giffords is scheduled to visit Capitol Hill on Tuesday, trying to rally support for the bill to expand background checks for people buying guns online and at gun shows. The gun debate will be underway on the Senate floor, aides said, but no major votes are expected Tuesday.

POLL: SUPPORT FOR GUN CHECKS STAYS HIGH. Today's ABC News-Washington Post poll found vast public backing for expanded background checks on gun sales, notes ABC News Pollster GARY LANGER. With a vote on gun control measures pending, 86 percent support extending background checks to gun sales at gun shows and online. Much smaller majorities support banning assault weapons and high-capacity magazines, 56 percent in each case, with the latter down from its peak, 65 percent in January, a month after the elementary school shootings in Newtown, Conn. A broad 72 percent back another proposal, prohibiting lawsuits against legal gun sellers if the weapon later is used in a crime. More generally, the public by 55-38 percent rejects the concept that any new gun control laws by definition limit gun rights, and by 52-40 percent says a higher priority now should be enacting new laws to try to reduce gun violence, rather than protecting the right to own guns. Still, another result reflects a gain across the past decade in positive attitudes toward guns: By 51-29 percent, Americans in this poll think having a gun in the home makes it a safer place to be, not more dangerous. (Sixteen percent say it depends.) When Gallup first asked this question in September 2000, the reverse held: Fifty-one percent said guns made homes more dangerous, 35 percent safer. http://abcn.ws/Zxyp5J

DC CELEBRATES EMANCIPATION DAY. The city of Washington today hosts events to commemorate the D.C. Compensated Emancipation Act, which officially abolished slavery in the nation's capital in 1862, notes ABC's ANJULI SASTRY. The act, signed into law by President Abraham Lincoln during the American Civil War, paid D.C. slave owners up to $300 to give up their slaves and ultimately freed more than 3,000 slaves, according to the U.S. Senate website. The act set a precedent for the Emancipation Proclamation that would come several months later, according to City Councilman Vincent Orange, who is the chair of the event's oversight committee. "It's the only time in history that the federal government paid $1 million in 1862 to free the slaves," Orange told ABC News. This year, the city will be honoring the day through parades and fireworks, and even a battle of local university bands and workshops on the day's history, organized by Orange. City employees and D.C. school kids also get the day off. http://abcn.ws/15hSdCh

IN THE NOTE'S INBOX

-TAX DOLLARS USED FOR LOBBYING? The government accountability group, Cause of Action, released a report today called: "CPPW: Putting Politics to Work", which the group says exposes the "lack of oversight and accountability within the Department of Health and Human Services and its Centers for Disease Control which led to the misuse of millions of taxpayer dollars by eight recipients of grants from the Communities Putting Prevention to Work program. Appropriated with $373 million under the American Recovery and Reinvestment Act of 2009, the CPPW program was intended for job creation and public education on tobacco use and obesity prevention. $94.4 million of the CPPW funds were allocated to grantees included in this report. CoA's nineteen-month long investigation revealed that CPPW money supported lobbyists and public relations companies who used taxpayer dollars to push laws and agendas that would lead to tax increases on tobacco and sugar sweetened products." REPORT: http://bit.ly/13aEkW2

-ON THE HILL: Charlie Drevna, President of the American Fuel and Petrochemical Manufacturers, will be testifying today in the House Natural Resources Committee in support of the Keystone Pipeline. A source e-mails The Note an excerpt of his remarks "After four years of extensive study and debate, it is clear building the Keystone XL pipeline would greatly benefit the United States. Pipelines are already the safest, cheapest and most reliable means of transporting crude oil and petroleum products. This pipeline will be the most advanced, state-of-the-art pipeline in use today exceeding all U.S. pipeline safety standards."

WHO'S TWEETING?

@TheWeek: One of the FBI's former top counter-terrorism agents talks with @marcambinder about the Boston bombings: http://bit.ly/10ZApG6

@BostonGlobe: Photo of 8-year-old Martin Richard of Dorchester, who was killed in the Boston Marathon bombings http://ow.ly/i/1U0Zc

@JoeBrettell: Classy gesture by @BDayspring and @Brook_H - today's NRSC Daybreak all about #Boston - no politics. Today we're Americans, not partisans

?@aterkel: Pro-gun group to protest outside Sen. Burr's office bc he allowed gun bill to go to floor for debate http://bit.ly/11lX40L

@michaelscherer: Dear North Korea, If you are going to attack someone without notice, best not to announce that: http://bit.ly/ZxC3MP