NEW YORK - The attack took just a few minutes. Times Square was aglow in the morning darkness but nearly deserted as a shadowy figure on a bicycle pedaled in and planted a small bomb that shattered the glass façade of the military recruiting station on Broadway.
The blast shook buildings and roused tourists from their hotel beds but caused little damage and no injuries. Still, the aftershock was felt all day. Streets and subway lines leading into and out of Times Square were closed for several hours. The FBI joined the New York Police Department in a search for witnesses and surveillance video and went back to the files to determine whether the bombing fit into a pattern of similar attacks, on the British and Mexican consulates, in 2005 and 2007.
"Any time there is an explosion of a bomb we have to be concerned, regardless of where it is," said the police commissioner, Raymond W. Kelly. "Certainly Times Square is the crossroads of the world, and we are concerned about that."
In Washington, the FBI and the Capitol police were investigating letters that were delivered yesterday to several congressional offices that contained a photograph of the Times Square recruiting station before it was bombed and a note saying: "We did it. Happy New Year."
Law enforcement officials confirmed that letters were received at fewer than a dozen offices, mainly those of Democrats. Security at the Capitol appeared to tighten, but by early evening, no lawmakers had come forward to say they had received one of the letters.
The letters, described by some officials as written on a greeting card, also contained a photograph of a person outside the recruiting station, officials said. It was not known whether the person was clearly identifiable. One official said the letter contained a return address.
Kelly said the authorities were trying to clarify video from a private security camera at 1501 Broadway, at 43d Street, that showed a timeline of the explosion and would perhaps yield clues about the bomber. With Times Square one of the most photographed and videotaped areas in the city, the police were "methodically going through" possible sources of other surveillance, Kelly said.
At 3:37 a.m., according to the security video, the bicyclist, barely illuminated by the marquee lights and neon signs of Broadway, pedaled to the station, reaching the door at 3:38. A few cars and trucks rumbled by, pinpointed in the darkness by their red taillights.
At 3:39, the bicyclist rode off.
Then, signs of the blast. At 3:40 on the video's time stamp, a fat cloud of white smoke billowed out at the entrance of the station and then rose into a thick column before wafting away, partially obscuring the flashing neon signs. The police said they responded at 3:43.
A man walking by on his way to buy a newspaper on 42d Street noticed a bicyclist outside the station acting "in a suspicious manner" just before the explosion, Kelly said. He heard the explosion, as did a police officer at the small police substation opposite the recruiting office.
The witness described the bicyclist as large, but he did not see the person's face, Kelly said.
The commissioner said that about four hours later, a 10-speed bicycle in good condition was found in or near trash receptacles at Madison Avenue and 38th Street, and investigators were checking to determine whether it was the same bike used in the bombing.
Streets and subway lines through Times Square were reopened around 6:30 a.m. to handle the thousands of cars and commuters arriving with the morning rush.
Hours after the blast, the pavement in front of the recruiting center was splashed with glass shards, which also clung to the window frame, revealing a glimpse of a poster of Uncle Sam. The adjacent door was ajar, its frame twisted.
Bashir Saleh, 51, a coffee vendor who works on the corner of 43d Street and Seventh Avenue, said he arrived at his spot about 3:45 a.m. "I was getting ready to set up the cart, and then I heard a very loud explosion," said Saleh. "It was the first time I ever heard such a thing."
A woman visiting from Florida, Mercy Sepulveda, said she heard the blast from the 11th floor of the Marriott Marquis hotel.
"I felt the building shaking," she said, "and then a second after, I heard the explosion. It sounded like a gas tank exploding."
Campaigning in West Palm Beach, Fla., Senator John McCain said, "My friends, a bad thing happened in Times Square this morning, and that is, some idiot tried to harm a recruiting station there in Times Square where we recruit men and women who serve in the military."
Senator Hillary Clinton, a Democratic candidate for president, struck a similar note in Washington. "I'm grateful that there were no injuries and minimal damage done," she said.