Authorities are investigating letters sent over the past 24 hours to Capitol Hill offices showing pictures of the Times Square military recruiting station that was bombed early Thursday morning.

The NYPD says it appears a small "low order" explosive device was thrown at the recruitment center just before 3:45 a.m. It did little damage, but the blast was significant enough to blow out the windows and door of the small center and to rattle a nearby hotel.

No injuries were reported and the mayor told reporters Thursday afternoon that there is no reason to believe it was an act of terrorism.

The letters, which were sent to the offices of several U.S. representatives, contained a ten-page political manifesto and what appeared to be a holiday card, but with photo of a man in front of the recruitment center coupled with the words "Happy New Year, We Did It."

The manifesto allegedly railed against the Iraq War. A memo was sent out to members of Congress alerting them of the letters and informing them that none of the letters contained any threats to members of Congress or the Congressional community.

Meanwhile, police are poring over Times Square surveillance footage in an effort to identify the person seen leaving the area prior to the early morning explosion.

During a briefing Thursday, Police Commissioner Ray Kelly said officials have few additional leads, but that the investigation is focused on a person who was spotted riding a bicycle in the area around the time of the blast.

Police say surveillance video shows a person on a bicycle in the vicinity at around 3:40 a.m. morning. Moments after, smoke is visible as the small scale explosive went off.

A new bike was found stashed not too far away from the scene on 38th Street, but officials aren't speculating if it was involved in the incident.

The police commissioner says a witness saw a person riding a bicycle "in a suspicious manner" just before the explosion. The individual was wearing a hood and dark colored clothing and had a backpack. The witness did not see the person's face, nor did the witness see the individual throw anything.

While the Joint Terrorism Task Force is participating in the investigation, Pentagon spokesman Bryan Whitman tells the Associated Press that, "We do get occasional vandals at our recruiting stations. It's unfortunate, but it happens from time to time."

New York's Junior Senator Hillary Clinton released a statement, commenting on the blast, saying in part:

"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."

Kelly noted several similarities between Thursday's blast and two other attacks within the city in recent history, but reiterated that it's still too early to say whether the three incidents are related.

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 were investigating whether it was connected to a similar incident at the British consulate on May 5, 2005, which took place while Britons were going to the polls to elect their prime minister.

In all three attacks, which occurred within the same time period between 3:30 and 3:50 a.m. on a Thursday or Friday, low-order explosives were tossed at the target from a person riding a bicycle.

Speaking out at a press conference Thursday morning, the mayor condemned the Times Square attack, calling the perpetrator a "coward" and promising to prosecute to the fullest extent of the law.

"We will not tolerate such attacks nor will we let them destroy our freedom to live peacefully and safely," said Mayor Michael Bloomberg.

Bloomberg said the attack was "deliberately directed at the recruiting center" and that it insults the efforts of the country's service men and women. The mayor said no calls have been made to the city claiming responsibility for the attack.

The police commissioner said that the city's arson and explosion unit is examining forensic evidence and that the device, which was encased in a small ammunition box, was not particularly sophisticated, but that it was powerful enough to have caused injury or even death.

"We're concerned and we're doing a very thorough investigation, working closely with the federal authorities," said Police Commissioner Ray Kelly.

Witnesses said they heard a bang and could feel the building shake.

"I heard the noise, I woke up, turned the TV on and I walked down here," said one area resident. "I've just been standing on the corner watching the action."

"Thank God I didn't have a long way to travel, to walk, but it's a mess out here, but thankfully it was four o'clock in the morning and not in four o'clock in the afternoon," added another.

Investigators are looking at video footage from the area's surveillance cameras, including those from surrounding hotels and small businesses.

The mayor said New Yorkers should not be intimidated by the attack, saying the city is "back and open for business," and that traffic has returned to Times Square.

Subway service has been restored after being briefly suspended in the area earlier Thursday morning.

New York military recruiters will be working temporarily out of their Union Square office, while the investigation continues.

The Pentagon has sent an alert to 1,600 military recruiting stations nationwide to use extra precaution.

Anyone with any information on the blast or who did it is asked to call the city's Crime Stoppers tip line 1-800-577-TIPS.