Although the 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 he released a video of a shadowy figure on a bicycle.

A law enforcement official said police are also investigating letters sent to congressional offices in Washington showing pictures of the centre.

About 10 members of Congress on Thursday received a letter with a manifesto and a photo of a man in front of the recruitment centre - before it was bombed - with a caption saying 'We Did It', sources and news reports said.

In an e-mail sent on Thursday night, US Capitol Police advised all members of Congress to be on the lookout for a manila envelope with two US$1 stamps on it that might contain the letter.

The blast, which occurred at about 3.40am on Thursday, left a gaping hole in the front window of the centre and shattered a glass door, twisting and blackening the metal frame of the building.

Nobody was injured.

A private security video, too grainy for police to get a clear description of the cyclist, shows a figure riding along a traffic island at 3.38am and getting off the bike just outside the centre. About two minutes later, the cyclist rides away. Then the explosion occurs.

The blast bears a striking resemblance to earlier attacks on the British and Mexican consulates.

In October last year, two small explosive devices were tossed over a fence at the Mexican consulate.

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.

ASSOCIATED PRESS, REUTERS