New York police are looking for clues as to who set off two explosive devices outside the building that houses the British consulate in Manhattan.
Police say the grenades were probably detonated by hand
The early morning blast shattered windows but did not cause any injuries.
The two devices, described by police as novelty hand-grenades packed with gunpowder, were put in a concrete flower tub in front of the building.
The area was cordoned off, but the British consul-general said he hoped to re-open the offices by the evening.
New York Mayor Michael Bloomberg said evidence from CCTV cameras was being checked.
Police studying the remains of the grenades say there is no evidence of a timing device and that they were probably detonated by hand.
Mr Bloomberg told reporters that although the British consulate was housed in the building on Third Avenue, it would be wrong to assume that this was the target of the attack.
"What exactly was targeted is still under investigation. We have no idea who did this or what the motive was, but eventually we will find out and apprehend them," Mr Bloomberg said.
Other countries have missions there and a number of domestic and international companies have offices in the building, the mayor said.
Nevertheless, with the explosion coming the same day as a general election in the UK, security was reportedly heightened at other British government offices in the US.
There was a heavy police presence outside the consulate in Washington and the street outside the mission in Chicago was closed off for half an hour as a precaution.
The explosions happened at about 0330 (0730 GMT). Mr Bloomberg said there had been no threats made prior to the attack, nor had anyone claimed responsibility since.
New York Police Commissioner Raymond Kelly said that other diplomatic locations in the city had been swept for devices as a precaution.
New York City has been on a "high" or orange state of alert since the 11 September 2001 attacks on the World Trade Center and the Pentagon, a level above that generally of the rest of the US, which is yellow or "elevated".
The alert level in the UK also stayed the same, sources told the BBC.
The force of the explosion threw a 30cm (one foot)-long chunk of concrete through the plate windows in the building entrance.
Police and firefighters in stations nearby rushed out when they heard the explosions, but did not see anyone running away, Mr Bloomberg said.
As emergency services responded, nearby roads and subway stations were closed, but the authorities say the subway stations have now reopened and that the roads will soon follow suit.
British Consul-General Sir Philip Thomas told PA news agency that he intended to re-open the consulate as soon as possible, and that a party planned to watch election results come in would go ahead.
"I am not frightened," Sir Philip said. "Clearly, this is election day in Britain and our staff just want to get on with their work."