U.S. law enforcement officials say there is no link between the bombing of a military recruiting center in New York City and letters sent to Congress containing a photo of the center with the message "We did it."

Law enforcement officials say the timing of the letters appears to be just be a coincidence.

New York City Police Commissioner Ray Kelly told CNN that the message "We did it" referred to the Democratic Party's takeover of Congress in 2006. 

Kelly said there was nothing "particularly threatening" in the letters, which were sent from Los Angeles and contained a photo with a man standing in front of the recruiting center.

The recruiting center was damaged Thursday in a pre-dawn bombing that shattered glass and buckled the station's front door.  The center was empty at the time and no one was injured.

The New York City police commissioner said a witness reported seeing an individual in hooded clothing and carrying a backpack and acting suspiciously near the center just before the explosion. 

The police commissioner said the blast is similar to attacks on the British consulate in 2005, and the Mexican consulate last year. 

The recruiting station is on a small traffic island in the center of Times Square, surrounded by theaters, restaurants and retail stores, in an area known as the "crossroads of the world."

Some information for this report was provided by AFP, AP and Reuters.