Letters saying "We did it" and containing a photo of the U.S. military recruiting station in New York that was the target of a bombing today were sent to as many as 10 members of Congress, law enforcement sources told ABC News.
A House aide said the letters were all received by Democratic members of Congress, and that all the letters were sent through the U.S. Postal Service.
The letters all had a return address in Hollywood, Calif., and authorities there are involved in the investigation, law enforcement sources said.
They contained the phrase, "Happy New Year -- we did it," and a lengthy statement against the Iraq war that could be described as a manifesto, sources said.
The small bomb caused minor damage to the recruiting station before dawn Thursday. Police have been searching for a hooded bicyclist seen on a surveillance video pedaling away.
No one was hurt in the explosion, but officials said the crude device could have "injured or even killed" someone.
However, officials say that because the bomb was detnated in the early morning hours when the recruiting office was empty and Time Square was mostly deserted, it is probable that the bomber did not intend to hurt anyone.
The concern is that the person, who clearly wants attention, might move on to actions in which people are harmed, and the person clearly has enough know how to do that, officials said.
High-level law enforcement sources told ABC News that a border stop in Canada has yielded evidence that may be linked to the case, and that individuals are being sought in connection with the case.
Canadian authorities declined to comment on the matter.
Sources said the border stop took place about a month ago, but today U.S. and Canadian authorities were revisiting the incident because of circumstantial evidence noticed at the time of the stop and the behavior of one of four individuals in the car that was stopped.
At the time of the stop, one young man fled the vehicle on foot, law enforcement sources said.
Evidence gathered during the stop included photographs of Times Square found in a backpack, and the material was given to U.S. authorities at the time, but an investigation at the time did not turn up any evidence of criminal activity, the sources said.
The surveillance video, which New York police released today, shows a bicyclist that matches the description provided by a witness who says he saw a suspicious man on a bike approach the recruiting station just before a small blast blew out the glass at the front of the building.
The recruiting office is not seen in the video, which the NYPD said came from a private security camera that was pointing just north of the building. But the camera captured a shadowy figure riding up the street on a bicycle, dismounting and walking across the street, then returning and riding off.
Just after the cyclist disappears, a flash of light and a large cloud of smoke drift into the frame after the explosion.
Mayor Michael Bloomberg vowed today to track down and prosecute those responsible.
"We will not tolerate such attacks," he said, adding that the apparent targeting of the famous recruiting station is "an insult to every one of our brave men and women serving around the world."
The device exploded around 3:45 a.m. No one was inside the recruiting center at the time.
New York City Police Commissioner Ray Kelly, who joined Bloomberg at a news conference this morning, said that the bomb was a "low-order explosive" made with powder placed in a green ammunition box.
"It was not a particularly sophisticated device," he said.
After the release of the video, Kelly said it was not clear if the grainy, poor quality video could be enhanced but added that investigators were still checking other video cameras in the area and hoped to find a better images.
No other eyewitnesses to the explosion have emerged, despite the fact that the recruiting station is directly across the street from a police substation and right in the middle of the famed "crossroads of the world."
The lone officer in the station heard the explosion and ran outside, Kelly said, but saw nothing. He did encounter a man who said he saw a man on a bicycle wearing a hoodie, dark clothing and a backpack acting suspiciously near the installation around the time of the blast.
The witness did not see anyone actually place the device or see the explosion, Kelly said.
Both the mayor and police commissioner emphasized that they had no evidence of any outside terrorist connection. There is "no evidence of any connection to anyone else," Bloomberg said.
The FBI is investigating the incident, along with the Joint Terrorism Task Force, which is made up primarily of FBI and New York Police Department officials. Any prosecution would likely draw federal charges since a military installation was targeted.
Tourists staying at a Marriott hotel four blocks away told The Associated Press they heard, and felt, the blast.
"It shook the building. I thought it could have been thunder, but I looked down and there was a massive plume of smoke, so I knew it was an explosion," said Terry Leighton, 48, of London, who was staying on the 21st floor of the Marriott.
Bloomberg, talking to reporters in a hectic Times Square, assured tourists and residents this morning, hours after the explosion, that the city was safe.
"New York City is open for business," he said.
Early in the investigation, subway cars passed through the Times Square station without stopping, but normal service was restored, with some delays, before the morning rush hour began.
The recruiting station, located on a traffic island surrounded by Broadway theaters and chain restaurants, has occasionally been the site of anti-war demonstrations, ranging from silent vigils to loud rallies.
"If it is something that's directed toward American troops that's something that's taken very seriously and is pretty unfortunate," Army Capt. Charlie Jaquillard, commander of Army recruiting in Manhattan, told the AP.
Although officals said they had no evidence to connect today's incident to earlier, similar incidents, the attack on the recruiting station did bear strong similarities to two past explosions in which small homemade bombs or incendiary devices were tossed at official buildings in New York City. Two were tossed at the British consulate in 2005 and two were tossed at the Mexican consulate last October.
In each case, a man on a bike was captured in hazy video images. Police found no second device this time.
This afternoon, Kelly said investigators had not made a link between today's blast and the other incidents, although he did say a bicycle was apparently used on all three occasions, and they all had similar low-impact explosions.
"We don't know what was used here, but we believe it was black powder used in the other two incidents," he said.
After the discovery of the letters, authorities in New York said it becomes a little more difficult to say whether the incident today is linked to the two earlier cases.
In all three, the devices were tossed just before 4 a.m., and each time black powder was used. There were no injuries and minor property damage -- broken windows, door frames and concrete flower pots in each of the incidents, and most importantly, in all three cases a suspect was seen either casing the scene on a bicycle, tossing the devices from a bicycle or leaving the scene on a bike.
Over the years, many protests have targeted the recruiting center.
In October 2005, a group of activists who call themselves the Granny Peace Brigade, rallied there against the Iraq War. Eighteen activists, most of them grandmothers — with several in their 80s and 90s — were arrested but later acquitted of disorderly conduct.
The recruiting station was renovated in 1999 to better fit into the flashy ambience of Times Square, using neon tubing to give the glass and steel office a patriotic American flag motif.
For a half century, the station was the armed forces' busiest recruiting center. It has set national records for enlistment, averaging about 10,000 enlistees a year.
ABC News' Dean Norland, Z. Byron Wolf, Jason Ryan, Dean Schabner and The Associated Press contributed to this report.