New York City police were questioning people and studying video surveillance tapes Thursday to determine who may be responsible for detonating two makeshift grenades outside the building that houses the British Consulate (search) earlier in the morning.

The explosions, which occurred on Britain's Election Day, caused the glass panels at the building's entrance in midtown Manhattan to shatter but no one was injured, officials said. A one-foot chunk from the planter was also torn off.

The blasts happened at 3:50 a.m. EDT.

"Nobody's reported seeing anybody, people heard several explosions … but we're questioning everybody," New York City Mayor Michael Bloomberg (search) told reporters at a morning press conference.

The FBI and police investigators were questioning a United Nations employee from the Netherlands who was found loitering near the building shortly after the explosion, FOX News confirmed. The man, Eric Van Schijndel, is a low-level employee at the U.N. Monitoring, Verification and Inspection Commission and is said to have been lingering near the building after the blasts. It's unclear whether Van Schijndel is a suspect.

The man -- who lives near the building -- was detained after he refused police orders to leave the crime scene, law enforcement officials told the AP.

Security was tightened at the United Nations and officials shut down several blocks surrounding the affected glass-and-metal office building, which also has stores, a bank and the New York offices of the Japanese newspaper Asahi Shimbun.

Workers were allowed in through a rear entrance about six hours after the explosion.

Bloomberg warned against assuming the explosives were aimed at anyone, or anything, in particular in that specific building. The British Consulate is located on the ninth and tenth floors of the building. There are other foreign and domestic offices located there as well.

"There is at the moment nobody claiming credit for this, there were no calls saying why, the explosion, who the target was ... there is absolutely no reason to jump to a conclusion that any one floor of that building was a particular target at this time," the mayor said.

New York Police Commissioner Ray Kelly (search) said the two novelty-type grenades had been placed inside one of 12 cement flower planters outside the front door of the midtown Manhattan building that houses the consulate. After piecing together the shrapnel, police determined the devices were toy grenades that had been filled with gunpowder. Officers estimated that one was the size of a pineapple; the other the size of a lemon.

"Let me stress — we have no known motive for this action at this time," Kelly said.

He said the two devices, which did not operate via a timer, could have been lit with the same fuse. Officials believe the two devices, while similar, were not identical.

Offices of other foreign diplomatic representatives were checked as a precaution and nothing was found, Kelly said.

At a later afternoon press conference, Kelly said video surveillance tapes showed a pedestrian on a bicycle riding north near the building on 3rd Avenue, as well as a taxicab passing in front of the building just before the blasts. He urged the cyclist, cab driver and anyone else who may have seen or heard anything suspicious to come forward with information.

In London, a Foreign Office spokeswoman, asked whether British authorities believed the blast was terror-related, said only: "Investigations are ongoing."

"We're not speculating about whether it's connected to the election," she added. Calls to the British Embassy in Washington were not immediately returned.

New Yorkers and visitors in general said they weren't phased by the explosions.

"There was no reason for me to feel unsafe in the area," said John Foust, an attorney who works about a block north of the explosion site. "It's important not to overreact. But that it is British election day strikes me as something more than a coincidence."

Stenstrom Goran and his wife, tourists from Stockholm, Sweden, were staying in the Doubletree Hotel, about a block away.

"We still feel safe coming to New York. We didn't expect it," he said. "But where are you safe?"

Britain's Foreign Office also said there were no provisions for Britons to vote at overseas consulates.

Britain's national elections have been dominated by anger with Prime Minister Tony Blair's support for the Iraq war.

The 14-story glass and metal building, on 3rd Avenue at 51st Street less than a mile from the United Nations headquarters, has retail shops on the lower level.

Authorities closed streets around the site, causing some rush hour disruptions. Trains on one subway line skipped the stop close to the site.

In Chicago, police closed a portion of Michigan Avenue near the British Consulate for about 30 minutes to search the area as a precaution, police spokeswoman Laura Kubiak said.

FOX News' Liza Porteus, Eric Shawn and The Associated Press contributed to this report.