Surveillance video trained on a military recruitment station in Times Square captured the image of a cyclist walking up to the door of the station moments before a homemade explosive detonated there early this morning, but police have not yet identified a suspect.

The video, stationed on a private building across the street from the traffic island at Broadway between West 43rd Street and West 44th Street, where the recruitment station is located along with a police department substation, shows a shadowy figure riding across the island just before 3:40 a.m.

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The person in the footage leans the bicycle against the railing of a ramp leading up to the military recruitment station, and then walks up to the door. After leaning down in front of the door, the figure then walks away, gets on the bike, and rides southward on Seventh Avenue.

Moments later, white smoke mushrooms from the front of the station, obscuring the lighted American flag on its side. Windows were shattered in the office, but no one was hurt.

Police recovered a portion of a tin ammunition box from the scene that they believe was used as a container for the explosives. The box was slightly larger than a phone book, and the portion recovered was printed with wording indicating that it was built to hold machine gun ammunition.

Police Commissioner Raymond Kelly said the box is one that would be available at most Army-Navy stores. Though “not particularly sophisticated,” Mr. Kelly said of the bomb, “this type of device could certainly cause injury and even death.”

Mr. Kelly said that the white smoke let off during the explosion indicated that the bomb was likely made with a low-grade explosive such as black powder. He said the low-grade nature of the explosives, along with the timing, were similar to two bombings in front of the Mexican and British consulates in the past three years.

“The explosives were roughly similar — the three of them,” Mr. Kelly said of the bombings.

Nearly three years ago, also on a Thursday, two grenades filled with black powder exploded in front of the British Consulate at around 3:50 a.m. In October last year, two grenades filled with the same sort of black powder were flung at the Mexican Consulate at around 3:30 a.m. Like today’s incident, a person riding a bicycle had been caught on surveillance tape riding “erratically” before both of the consulate explosions, Mr. Kelly said.

No suspects have been arrested in any of the previous bombings, and the origins of the grenades have not been traced.

At a morning news conference in Times Square, Mayor Bloomberg said that officials were “not in the business of speculating” whether the three incidents were linked, but added: “It’s a similar kind of thing — a small bomb, a small explosive device going off outside of a building, where nobody happened to see the perpetrator.”

“There is no connection to anybody else. That’s all we know and we don’t want to get ahead of the story here,” Mr. Bloomberg said.

This afternoon, Mr. Kelly said that investigators have been talking to a witness who said he saw a large person riding a bike on the traffic island moments before the explosion. Mr. Kelly said the witness said the person had been riding “in suspicious manner.” The person was wearing a hood, dark colored clothing, and a backpack, and had ridden onto the traffic island where the military station stands, Mr. Kelly said.

“He did not see this individual’s face,” Mr. Kelly said of the witness. “He was not able to give us a complete description.”

After the explosion, the witness returned to the scene, where he met with a police officer coming out of the neighboring substation to respond to the incident, Mr. Kelly said. Several other people were visible in the footage walking across the island just before the bomb went off, and police said they were seeking to speak with them.

This morning at around 7 a.m., Mr. Kelly said a superintendent at a building at East 38th Street and Madison Avenue reported that he had found a 10-speed bicycle in the trash. Police said the bicycle seemed relatively new, and that investigators were examining it to determine if it was linked to the Times Square bombing.

After the explosion, police flooded Times Square, the symbolic heart of the city and one of its major transportation hubs.

Police said they were investigating this morning’s explosion jointly with the FBI, and that evidence collected at the scene — including the side of the ammunition box — had been sent to FBI labs at Quantico, Virginia. He said police were also looking at footage from other surveillance cameras in the area, including two NYPD cameras located in Times Square, in the hopes of finding a clearer image of the suspect. He said they were also canvassing local hotels for other witnesses, and asked for anyone who might know something to come forward and speak to police.

The sidewalk in front of the military office has often been the scene of protests.

The commander of U.S. Army recruiting operations in Manhattan, Captain Charlie Jaquillard, told the Associated Press this morning that no one had been inside the building at the time of the explosion. He said two noncommissioned officers are usually stationed there during the day.

“If it is something that is directed toward American troops, then it’s something that’s taken very seriously and is pretty unfortunate,” he was quoted as saying.

A second class petty officer in the Navy, Miguel Cedeno, was leaving the station this morning at around 11 a.m. after removing some paperwork there. He has worked at the station since May, he said, and enlisted in the Navy at the station six years ago.

“I was really surprised. I got a call at five in the morning from a friend who was checking to see if I was okay,” he said. “We don’t come in until 7 a.m. I hope it’s up and running soon.”

News of the explosion quickly prompted reactions on the presidential campaign trail this morning.

Senator McCain, the Republican nominee, said: “We cannot allow this to happen to the men and women serving in our military whether they are at home or abroad.”

Senator Clinton, also said she was “deeply concerned” that the attack had targeted the military.

“While we should be grateful that there were no injuries and minimal damage, there is an ongoing investigation into whether the attack is linked to foreign terrorist groups, and federal, state, and city authorities should be given every resource and every tool to swiftly complete that investigation,” she said.

Subways that run through Times Square were delayed this morning following the explosion, but are now running.

“New York City is back and open for business,” the mayor said. “People are going about their business, shopping, working, and sightseeing. They are not intimidated. Whoever the coward is that committed this disgraceful act against our city will be found and prosecuted to the full extent of the law.”

“This city is safe,” he added.