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.