Police are attempting to determine if a bomber on a bicycle who detonated a homemade explosive device in front of a Times Square military recruitment station yesterday is linked to two previous bombings at foreign consulates in the city.

In the earlier bombings at the Mexican Consulate last year and the British Consulate in 2005, a bicyclist was caught on surveillance video flinging grenades full of low-grade explosive powder. The powder in the previous incidents is similar to what was likely used in the Times Square bomb yesterday.

Surveillance video from a camera set up next to a police substation across the street from the military station, , which is situated on a traffic island at Broadway between 43rd and 44th streets, captured the image of the cyclist riding up moments before the explosion at 3:43 a.m.

In the footage, a figure is seen leaning the bicycle against the railing of a ramp leading up to the military recruitment station and walking up to the door. After bending down in front of the door, the person then gets on the bike and rides southward on Seventh Avenue.

Seconds later, white smoke mushrooms from the front of the station, billowing over a neon American flag on the building’s side. Windows were shattered in the office, but no one was hurt.

Commissioner Raymond Kelly said investigators recovered a portion of a tin machine-gun ammunition box from the scene that they believe was used as a container for the explosives. The box, slightly larger than a phone book, would be available at most Army-Navy stores, police said.

Although “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 nature of the explosives, along with the pre-dawn timing, were “roughly similar” to the consulate bombings.

No suspects have been arrested in the previous bombings.

At a morning news conference in Times Square, Mayor Bloomberg said he would not speculate whether the three bombings 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.”

Police located a witness who said he saw a large person riding a bike “in suspicious manner” on the traffic island moments before the explosion. He told police the cyclist was wearing dark clothing and a backpack, and said the person’s face was obscured by a hood. Police are also investigating whether a 10-speed bicycle found in the trash at a building on East 38th Street and Madison Avenue was linked to the Times Square bombing. The bicycle is being examined by police in New York, while post-blast evidence collected at the scene was sent to FBI labs at Quantico, Va.

The sidewalk in front of the military office has often been the scene of protests against the war in Iraq. No groups had sought credit for bombing as of yesterday evening. Mr. Kelly said police are investigating whether the date of the explosion had any significance.

On the same date in 1970, an explosion ripped apart a townhouse in Greenwich Village that was a safe house for the Weathermen, a leftist group, as the group prepared an improvised explosive device that was to be set off at Fort Dix to protest the Vietnam War. Military personnel, the mayor, and presidential candidates on the campaign trail expressed dismay that the military was targeted.

After the explosion, police flooded Times Square, the symbolic heart of the city and one of its major transportation hubs. Subways that run through Times Square were delayed after the explosion but were running a few hours later. “New York City is back and open for business,” the mayor said. “This city is safe.”