6:34 a.m. ET: ABC News has confirmed that at least 145 are injured following the Boston Marathon attack and 17 of those are critical injuries.
6:15 a.m. ET: ABC News' Boston affiliate WCVB reports the 8-year-old victim has been identified as Martin Richard. A single candle was placed in front of his home overnight in Dorchester, Mass.
1:30 a.m. ET: Teams of federal and local law enforcement agents searched an apparently vacant fifth floor apartment on Ocean Avenue in the Boston suburb of Revere just hours after a pair of bombings at the Boston Marathon, residents told ABC News.
12:47 a.m. ET: Sources tell ABC News authorities are looking for any friends and associates of the 20-year-old Saudi national.
12:07 a.m. ET: ABC News' Aaron Katersky reports the working theory about the bombs, according ti Massachusetts law enforcement, is that they were small, crudely made devices hidden in bags or backpacks, planted either during the race or immediately prior and detonated remotely, possibly with a cell phone. Officials here have subpoenaed cellphone records.
11:50 p.m. ET: ABC News can confirm that one of the people law enforcement officials are talking to is a 20-year-old Saudi national at a Boston hospital. Sources tell ABC News that he is here legally on a student visa and that his visa is clean with no apparent criminal history.
Boston police tonight, at a news conference, said that there are people they are talking to, but no suspects.
10:35 p.m. ET: Ron M. Walls, chairman of Boston's Brigham and Women's Hospital, told reporters that the hospital has 29 total patients, 8 of whom are in critical condition. Walls said that the only shrapnel they have seen in injured victims is material from the street - bits of metal, soda cans and anything that was really close to a blast and became fragmented. He said there have been no reports of ball bearings or anything that seemed as if it may have been placed into the explosion to cause more damage.
9:27 p.m. ET: ABC News has revised its estimate of the total number of patients initially sent to hospitals to at least 133.
9:17: p.m. ET: Boston Police Commissioner Ed Davis : "I offer my sympathies to the families impacted by this horrible tragedy. Those responsible will be brought to justice."
8:56 p.m. ET: The Boston Police Department has said that it does not have a suspect in custody.
8:54 p.m. ET: Three people have died, according to a tweet from the Boston Police Department.
8:03 pm ET: At least 115 people injured in Boston Marathon bombing, according to law enforcement officials.
7:22 p.m. ET: An 8-year-old child is among those killed in the Boston bombings, a law enforcement source confirmed to ABC News.
6:41 p.m. ET: ABC News has revised its estimate of the number of people injured in the Boston Marathon bombing incident to at least 99 people.
6:29 p.m. ET: Boston hospitals report "multiple amputations" among those injured at Boston Marathon.
6:13 p.m. ET: "We will find out who did this and we will hold them accountable," says President Obama.
6:12 p.m. ET: We do not know who did this or why… But make no mistake. We will get to the bottom of this," says President Obama.
6:11 p.m. ET: "On days like this there are no Republicans and Democrats," says President Obama.
6:10 p.m. ET: President Obama is now addressing the nation.
6:04 p.m. ET: Sen. Dianne Feinstein, D- Calif., chairman of the Senate Intelligence Committee, tells ABC News: " It is a terrorist incident. … It could be foreign, it could be homegrown." She said the attack had the "hallmarks" of a terror attack.
6:02 p.m. ET: ATF agents have descended on Brigham and Women's hospital where they're questioning one potential person of interest.
5:56 p.m. ET: At least 86 people have been taken to area hospitals, according to a tally by ABC News.
5:48 p.m. ET: FAA lifts restrictions on air travel over Boston. Logan Airport has been reopened.
- Boston Police Dept. (@Boston_Police) April 15, 2013
5:41 p.m. ET: President Obama expected to address the nation at 6:10 p.m.
DRAMATIC NEW VIDEO:
5:37 p.m. ET: Boston police now say library incident was likely an unrelated fire.
5:28 p.m. ET: Red Cross says it does not need any more blood donations. "There is currently enough blood on the shelves."
5:22 p.m. ET: Federal law enforcement officials are treating incident as a "terrorist attack."
4:58 p.m. ET: No one has taken responsibility for the attack, says Police Commissioner Ed Davis.
4:55 p.m. ET: There was also a controlled explosion on Boylston St., not one of the three incidents, says Commissioner Ed Davis.
4:53 p.m. ET: Police ask anyone with knowledge of attacks to call 1-800-494-TIPS
4:51 p.m. ET: Determined there was a third incident at JFK Library around 4:20 p.m. Not certain if incidents are related, but treating them as if they are, says Davis. There are no known injuries from that event.
4:50 p.m. ET: Two explosions, 50 to 100 yards apart at the finish line at 2:50 p.m. "All victims have been removed from scene," Boston Police Commissioner Ed Davis said at a news conference.
4:49 p.m. ET: New York City Mayor Michael Bloomberg says security increased throughout that city.
4:48 p.m. ET: Massachusetts authorities confirm at least 68 patients transported to area hospitals.
4:41 p.m. ET: Boston Police are asking the public to turn over any video of the finish line explosion.
4:38 p.m. ET: "There is no sign of any radiological material involved in the explosion," a federal source tells ABC News.
4:35 p.m. ET: Two more explosive devices have been found near the scene of the explosion, according to the Associated Press.
4:34 p.m. ET: Some 400 Massachusetts National Guard soldiers were on the scene for the marathon.
4:24 p.m. ET: A Federal law enforcement source confirms to ABC News this was an intentional bombing, using small portable explosive devices.
4:21 p.m. ET: Boston police confirm they have found three more suspicious devices. One has been rendered safe.
4:13 p.m. ET: The FAA has issued a temporary flight restriction over the area of the explosion.
4:11 p.m. ET: Boston Fire Department to brief media at 4:30 p.m. at Westin Hotel.
4:09 p.m. ET: Tufts Medical Center confirms that 9 victims have been taken there.
4:06 p.m. ET: Law enforcement authorities tell ABC News they are testing smoke and materials at the scene for chemical substances.
4:05 p.m. ET: Boston police confirm at least 22 injured, 2 dead.
- Boston Police Dept. (@Boston_Police) April 15, 2013
4:03 p.m. ET: Police to search hundreds of bags left at scene by marathon participants.
4:02 p.m. ET: "The president has been notified of the incident in Boston," a White House official tells ABC News. "His administration is in contact with state and local authorities. He directed his administration to provide whatever assistance is necessary in the investigation and response."
3:55 p.m. ET: Police set off controlled explosion in Boston. Third explosion heard.
3:54 p.m. ET: Marathon's website tracking runners who completed the race, here
3:43 p.m. ET: Trauma nurse tells ABC News that the race's medical tent has become a makeshift morgue. Dealing with injuries including severed limbs and children with severe burns.
3:41 p.m. ET: Eyewitness Hayden Cary describes a "hectic, crazy scene." Heard explosion go off within seconds of each other near a car, steps from finish line.
3:34 p.m. ET: Police bomb squads sweep area looking for suspicious packages.
3:29 p.m. ET: Massachusetts General Hospital confirms at least 4 injured taken there.
3:25 p.m. ET: Police warning people in area to avoid trash cans.
3:15 p.m. ET: Boston Police Department says two explosions on Boyleston street near the finish line of the Boston Marathon. Police, fire and bomb squad responding. The number of injuries is unknown. The cause is unknown.
The two explosions took place around 2:56 pm, within second of each other. The first runners in the marathon crossed the finish line around noon.