Mark Lennihan, Associated Press
U.S. soldiers walk on broken glass at a recruiting station that was bombed in New York's Times Square on Thursday.

NEW YORK — For the third time in as many years, someone riding a bike and armed with a small explosive has struck in Manhattan, this time in the highest-profile location by far: a landmark military recruitment station in the heart of Times Square.

The bomb, contained in a metal ammunition box, produced a sudden flash and billowing cloud of white smoke at about 3:40 a.m. Thursday — a scene captured by numerous security video cameras. When the smoke cleared, there were no injuries, serious damage or clear indication of motive.

But like similar attacks on the British and Mexican consulates, the explosion frayed nerves of New Yorkers and tourists alike. Although authorities have not definitively linked the three attacks, the latest episode heightened speculation that they were the work of a lone bomber who, perhaps emboldened by his past success, sought out the bright lights of Times Square.

"Times Square is 'the crossroads of the world,' and we're concerned about it," Police Commissioner Raymond Kelly said at a news conference where the NYPD released a video of the shadowy figure on a bicycle.

A law enforcement official said police are investigating letters sent to Capitol Hill offices showing pictures of the recruiting station. According to the official, who was briefed on the investigation, the letters included words to the effect of, "We did it." The official did not know which offices received the letters.

The Associated Press obtained a copy of an e-mail from the office of Sen. Dianne Feinstein, D-Calif., to other lawmakers Thursday that reads:

"A few offices on the House side have received a letter today addressed to 'Members of Congress' with a picture of a man standing in front of the Times Square recruiting station that was bombed in New York today with the statement 'We did it.' He is standing in front of it with his arms spread out, and he's attached his political manifesto."

The e-mail advised recipients to leave the letters alone and call police.

The blast prompted a huge police response, left a gaping hole in the front window and shattered a glass door, twisting and blackening the metal frame of the building. Guests at the nearby Marriott Marquis said they heard a "big bang" and felt the building shake.

"I thought it could have been thunder," said Terry Leighton, 49, a Londoner staying on the 21st floor. "I looked down and there was a massive plume of smoke."

The private security video, though too murky for police to get a clear description of the cyclist, shows a figure riding along a traffic island in the glow of neon signs at about 3:38 a.m and getting off the bike just outside the recruitment center. About two minutes later, the cyclist rides away. Then the explosion occurs.

Investigators were studying other security videos, including one showing a man exiting a subway station about 10 blocks away carrying a bicycle, police said. The FBI was analyzing forensic evidence collected at the scene, Kelly said.

The commissioner cited other possible clues: A new bike discovered at about 7 a.m. in a Dumpster just a few blocks from the blast, and the sighting of a man spotted on a bike near the scene moments before the explosion.

The man caught the attention of a witness because he was riding slowly, wearing a backpack and a hooded jacket, Kelly said. The witness, who was buying a newspaper at the time, said because of the hood, the rider's face "was pretty much covered."

The blast bears a striking resemblance to the two consulate explosions.

In October, two small explosive devices were tossed over a fence at the Mexican consulate, shattering some windows; police said they believed someone on a bicycle threw the devices.

At the time, police said they were investigating whether it was connected to a nearly identical incident at the British consulate on May 5, 2005. No one was arrested in either incident.

In those incidents, "dummy hand grenades were used and the explosive, black powder, was put into those grenades and that caused the explosion," Kelly said. "Here, it may be similar powder — we still have to determine that. But it was placed in an ammunition box. That was the carrier for the explosive. There was no grenade."

Kelly held up a similar green metal box, noting they were readily available in Army-Navy surplus stores.

In another sign that the three blasts are related, all of them occurred between 3:30 a.m. and 4 a.m.

But the previous episodes generated nowhere near the response that the Times Square blast did. Kelly, Mayor Michael Bloomberg and the top FBI official in New York appeared at a nationally televised news conference in Times Square, and presidential candidates issued statements condemning the blast.

Bloomberg said the act "insults every one of our brave men and women in uniform stationed around the world."

"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. "We will not tolerate such attacks."

The military's 1,600 recruiting stations nationwide were alerted and advised to use extra caution, said Douglas Smith, spokesman for the Army recruiting command. He said New York recruiters would be working temporarily out of their Union Square office.

The military has had a recruiting presence in Times Square since the 1940s.

The current version of the station, built around eight years ago for $1.5 million, was designed to fit into the revitalized Times Square area, with American flags in neon lights and video screens offering recruiting messages and other information.

Over the years, the center's high-profile location made it a favorite target of anti-war demonstrations.