New York City police shuts down portion of Third Avenue in Manhattan as they investigate explosion in front of building which houses British Consulate, Thursday

New York police are reviewing surveillance tapes, looking for clues after two home-made grenades exploded early Thursday outside a New York City office building that houses the British Consulate, as well as other offices. Police are looking for witnesses and a motive.

Two crude grenades stuffed with black powder exploded at 3:30 a.m., shattering glass, tearing a chunk from a concrete flower pot, but causing no injuries.

New York City Police Commissioner Raymond Kelly says investigators are still reviewing 17 video surveillance tapes from the vicinity. "We do have a tape, it is of not particularly good quality, some have interpreted as showing one of the grenades may have been thrown. It may, in fact, have been thrown or one of the grenades may have been embedded in the planter," he said.

Commissioner Kelly reiterated that the crude devices were possibly ignited by hand. He said the top of one of the grenades with a fuse was found on the ninth floor of a building across the street.

He adds that surveillance tapes show several people, including a jogger, in the vicinity at the time of the explosion. Authorities do not have any eyewitnesses or a motive.

The British consulate is one of several offices located in the building. "Obviously, one of the things put forward is the fact there were elections today in the U.K. That is obviously something we're considering. We are looking at all the tenants in the building," he said. "There was a demonstration there, I believe on April 13 that had to do with we believe the fact that an individual in that building is a member of the board of Caterpillar tractors. And this demonstration was focused on the fact the tractors used to knock down homes in the Palestinian area, in Gaza."

At an earlier news conference, New York City Mayor Michael Bloomberg cautioned people not to jump into conclusions. "We do not, at this point, have any idea who did it, or a motive, but we have the best people working on investigating, and I think, eventually, we'll find out who did it, and we will apprehend them," he said.

Commissioner Kelly says a U.N. employee from the Netherlands has been ruled out as a suspect. The U.N. employee, who lives near the building, had been arrested for failing to comply when officers told him to stay away from the crime scene.