New York City police officers and firefighters cordoned off much of Times Square for more than two hours after a small explosion — set off, the authorities said, by an “improvised explosive device” — damaged the front of the Armed Forces Career Center on the traffic island bounded by 43rd and 44th Streets, Seventh Avenue and Broadway at 3:43 a.m., officials said. No one was injured, and after a temporary interruption, subway service was restored.

Most traffic around Times Square was allowed to pass by 6:45 a.m., after vehicles had been diverted for more than two hours. City officials confirmed that police had initially blocked off the area as a precaution to ensure that there was no secondary device or other threat; the officials emphasized that they did not believe anyone was in danger.

Police officers at the scene said the explosion blew a hole through the front door of the recruiting station, which is at the northern end of the structure.

Members of the Joint Terrorism Task Force, the large Police Department and F.B.I. unit that investigates terrorism, were at the scene of the blast, supporting the Police Department’s Bomb Squad, which along with other police detectives likely will take the lead role in investigating the incident, an F.B.I. official said. The official said that in today’s attack, a man in a gray hooded sweatshirt was seen leaving the scene on a bicycle.

The authorities were looking into whether the explosion was connected to two earlier blasts that were similar in method and timing, the official said. At about 3:40 a.m. on Oct. 26, 2007, two dummy hand grenades that had been fashioned into crude bombs exploded outside the Mexican Consulate at 27 East 39th Street in Murray Hill, shattering windows. The building was not occupied and no one was hurt. At 3:55 a.m. on May 5, 2005, two crude but powerful explosive devices detonated outside the British Consulate at 845 Third Avenue in East Midtown, shattering windows and damaging a planter.

Mr. Kelly spoke with Mayor Michael R. Bloomberg at a news conference at 9:30 a.m. in Times Square. Later, at a second news conference, Mr. Kelly said the explosive material used today “was not particularly powerful,” and that the explosion was “roughly similar” to the earlier consular bombings. Material from the two earlier explosions has been sent to the F.B.I. crime lab in Quantico, Va., and material from today’s explosion will be taken there for analysis as well, Mr. Kelly said.

A witness who was buying a newspaper told police that he saw a man wearing a gray-hooded sweatshirt biking around the recruiting center moments before the bombing. After the bombing, at about 7 a.m., a building superintendent notified the police that a 10-speed bicycle in good condition had been abandoned at Madison Avenue and 38th Street. The police are looking into the possibility that the bicycle had been used by the attacker.

From the writing on the side of the ammunition box found at the site of the explosion, it appeared that the box had once contained belt-mounted machine-gun ammunition. The box was 4 inches wide, 10 inches long and 10 inches deep, Mr. Kelly said.

In the two earlier incidents, the police said the improvised explosive devices were virtually identical. Both contained explosive powder. Both attacks occurred before dawn when there were few people on the street. And in both cases the devices were thrown against building facades.

Both of those attacks were captured on video surveillance. The arc of the device — with its lighted fuse – was visible on the videotape. In the 2007 attack, a witness told police that a man in his 20s on a bicycle pedaled quickly away and turned south onto Park Avenue, the authorities said at the time. The bicyclist was wearing a hooded gray jacket and his face was partly covered, but the police could not say for certain that he threw the grenades. At the time of the Mexican consulate attack, Mr. Kelly, the police commissioner, described the two improvised explosives. He said they were training, or dummy, grenades — perhaps purchased from a novelty shop — that had been hollowed out and stuffed with gunpowder, possibly black powder, and equipped with pyrotechnic fuses. One had a smooth surface, modeled after the “lemon” type used in the Vietnam War, and the other was scored like the rough “pineapple” type used during World War II.

Although the damage today was relatively minor, the prominent location of the blast at a time of heightened concerns about terrorism attracted national attention.

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