NEW YORK – A small bomb caused minor damage to an empty military recruiting station in Times Square early Thursday, shaking guests in hotel rooms high above “the crossroads of the world.”

The blast, which happened around 3:45 a.m., left a gaping hole in the front window and shattered a glass door, twisting and blackening its metal frame. No one was hurt. But Police Commissioner Raymond Kelly said the device, though unsophisticated, could have caused “injury and even death.”

A witness saw a person on a bicycle wearing a backpack and a hood and acting suspiciously, but no one saw the device being placed in front of the recruiting center, authorities said at a news conference.

“If it is something that’s directed toward American troops then it’s something that’s taken very seriously and is pretty unfortunate,” said Army Capt. Charlie Jaquillard, who is the commander of Army recruiting in Manhattan.

He said no one was inside the station, where the Marines, Air Force and Navy also recruit.

Witnesses staying at a Marriott hotel four blocks away said they could feel the building shake with the blast.

“It shook the building. I thought it could have been thunder, but I looked down and there was a massive plume of smoke so I knew it was an explosion,” said Terry Leighton, 48, of London, who was staying on the 21st floor of the Marriott.

David Hassan, who operates a breakfast cart across the street, said he was in the area when the explosion happened.

“I saw smoke and there was a loud boom,” he said. “I was scared. I’m still shaky.”

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.

Pentagon spokesman Bryan Whitman said no official higher state of alert had been issued. “We do get occasional vandals at our recruiting stations,” Whitman said. “It’s unfortunate but it happens from time to time.”

New York recruiters will work out of their Union Square office for now, he said.

Members of the police department’s bomb squad and fire officials gathered outside the station in the early morning darkness, and police cars and yellow tape blocked drivers – most of them behind the wheels of taxicabs – from entering one of the world’s busiest crossroads. Police began allowing some traffic through around the start of rush hour.

Authorities were still trying to determine exactly what kind of device was used. When investigators went through the evidence, they found a metal ammunition box that is believed to have contained the explosive. It was being sent for testing. Kelly said the box was readily available in Army-Navy surplus stores.

“Whoever the coward was that committed this disgraceful act on our city will be found and prosecuted to the full extent of the law,” Mayor Michael Bloomberg said.

The recruiting station, located near the theater district on a traffic island, is surrounded by chain stores and restaurants and several TV studios, has occasionally been the site of anti-war demonstrations, ranging from silent vigils to loud rallies.

In October 2005, a group of activists who call themselves the Granny Peace Brigade rallied there against the Iraq war. Eighteen activists, most of them grandmothers with several in their 80s and 90s, were later acquitted of disorderly conduct.

The recruiting station was renovated in 1999 to better fit into the flashy ambiance of Times Square, using neon tubing to give the glass and steel office a patriotic American flag motif. For a half century, the station was the armed forces’ busiest recruiting center. It has set national records for enlistment, averaging about 10,000 volunteers a year.

Police said it was too early to say if the blast may have been related to two other minor explosions in the city.

In October, two small explosive devices were tossed over a fence at the Mexican consulate, shattering three windows but causing no injuries. No threats had been made against the consulate, and no one took responsibility for the explosion, police said.

At the time, police said they were investigating whether it was connected to a similar incident at the British consulate on May 5, 2005.

In that incident, the explosions took place in the early morning hours, when Britons were going to the polls in an election that returned Prime Minister Tony Blair to power.

In both cases, the instruments were fake grenades sometimes sold as novelty items. They were packed with black powder and detonated with fuses, but incapable of causing serious harm, police said.

Associated Press Writers Ula Ilnytzky in New York and Pauline Jelinek in Washington contributed to this report.