Letters after Times Square bombing claim: 'We did it'

NEW YORK (CNN) -- Letters claiming responsibility for the bombing of a military recruiting station in New York's Times Square arrived in the Washington offices of several members of Congress on Thursday, just hours after the blast, authorities said.

Police are searching for a hooded bicyclist seen on a surveillance tape just before an early morning bombing at the office, according to the FBI.

The letters sent to Capitol Hill contained at least one picture of the station, apparently before the attack, a law enforcement official familiar with the investigation told CNN.

Police knew of fewer than 10 of the letters that had been received by members of Congress, a second law enforcement source said.

The letters were all received by Democrats, another law enforcement source said.

They contained a picture of a man standing in front of the recruiting station with the statement "We did it," according to an e-mail sent by the office of Sen. Dianne Feinstein, D-California, and obtained by CNN.

"He is standing in front of it with his arms spread out and he's attached his political manifesto," the message said.

An e-mail sent to members of Congress from Capitol Police described the letters as arriving in 5-by-8-inch manila envelopes. The e-mail warned recipients not to open them, but to contact police immediately.

Capitol Police said that they were working with the FBI to investigate, but that there was no "established connection" between the letters and the blast.

Authorities consider the bicyclist in the surveillance video a possible suspect in the bombing, which occurred about 3:40 a.m. and caused no injuries.

The video, shown by Police Commissioner Ray Kelly, shows the person approach the station on the bicycle, get off and walk up a metal ramp to the door.

After about a minute, the person returns to the bike and rides away at 3:39 a.m.

A minute later, a bright flash and a cloud of white smoke signal the explosion.

A witness at the scene saw the person on the bicycle, but couldn't distinguish sex or race because the individual was wearing a hooded jacket that "covered most of the face," Kelly said.

However, police suspect the cyclist was male because the witness said the person was large.

A 10-speed bike in good condition was later found in a trash bin not far from the scene. Police said they were trying to determine if it was the bicycle seen in the video. Watch surveillance video of the bicyclist at the bomb site »

The bomb consisted of some kind of "low-order explosive" contained in a military-style ammunition box, Kelly said.

"This was not a particularly sophisticated device," he said.

Mayor Michael Bloomberg was defiant in the wake of the blast.

"Whoever the coward was that committed this disgraceful act on our city will be found and prosecuted to the full extent of the law," Bloomberg said. Watch the mayor and commissioner discuss the case »

"The fact that this appears deliberately directed at the recruiting station insults every one of our brave men and women in uniform stationed around the world fighting to defend our freedoms and the things we hold so dear," Bloomberg said.

After the incident, the Army sent an alert to the 1,650 Army recruiting stations across the country, said Douglas Smith of the U.S. Army Recruiting Command.

The blast is similar to two other incidents in New York, one in October and one in May 2005.

Both times, an explosive device was detonated around 3 a.m.

In the October incident, a bomb was detonated near the Mexican Consulate, shattering windows. At that time, authorities said the explosive device was similar to the ones used in a May 2005 blast at the British Consulate.

In both cases, authorities were looking for a person spotted riding a bicycle in the area.

Police stopped several bicyclists for questioning Thursday morning near Times Square, but no one was detained, WABC-TV reported.

The recruiting office, whose neon lights help it blend in with the bright atmosphere of Times Square, sits on a triangular traffic island across the street from the Hard Rock Cafe. Map »

Two witnesses, a man and a woman, told a local TV station they heard a "huge bang" from their hotel rooms, but there were no police or fire reports of serious damage or injuries.

"I was on the 44th floor, and I heard it. I could feel it from up there," a woman told WABC-TV. Watch what witnesses say »

The office's front door and window were shattered by the explosion, and the door's metal frame was bent. A large television screen above the door was working intermittently.

Investigators from the New York police and fire departments as well as the federal Bureau of Alcohol, Tobacco, Firearms and Explosives sifted through the shattered glass by hand, placing small pieces of material into evidence bags.

The evidence was sent to the FBI's crime laboratory in Quantico, Virginia, Kelly said.

Police and fire squads initially cordoned off part of an area around Times Square. Later, vehicular and foot traffic resumed in the area, but the traffic island was closed to pedestrians.

Subway service to Times Square was briefly halted but resumed in time for the morning rush hour.

Recruiters will work from an office in Union Square until repairs are made, Pentagon spokesman Bryan Whitman told AP.

"New York City is back and open for business," the mayor said. To prove his point, Bloomberg walked across the busy street after the news conference and bought a cup of coffee at a cafe. E-mail to a friend

CNN's Barbara Starr and Kevin Bohn contributed to this report.

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