After a pair of World War II era grenades exploded early yesterday outside the Midtown building housing the British Consulate, the police commissioner, Raymond Kelly, said detectives had not isolated a motive behind the explosions.

No one was reported injured and damage was minimal in the blasts, which went off around 3:30 a.m. at 845 Third Ave. If the grenades had been detonated during business hours, Mr. Kelly said, the explosions could have caused significant injuries.

While yesterday’s elections in Britain suggested the grenades might have been politically motivated, perhaps in response to the Blair government’s role in the war in Iraq, Mr. Kelly said detectives were also probing an activist group, Jews Against the Occupation, that conducted a political demonstration last month outside the same office building. The group organized a protest April 13 outside the building against the industrial manufacturer Caterpillar, saying the company was violating human rights by allowing the Israeli government to buy its bulldozers for use in conjunction with military activities in the West Bank and Gaza. The target of last month’s protest was a Caterpillar board member, Gail Fosler, who is chief economist at a nonprofit economic-research organization, the Conference Board, which has offices at 845 Third Ave.

A U.N. employee from the Netherlands was taken into police custody yesterday for questioning. Mr. Kelly said the employee had been in the vicinity of the building at the time of the blasts. Shortly afterward, as crime scene investigators and officials from the Police Department’s bomb squad began cordoning off the area with yellow tape to sweep for other devices and gather forensic evidence, Mr. Kelly said, the man returned and sought to get past police investigators.

The man was identified by police as Eric Van Schijndel, 40, who was arrested at about 4:10 a.m. for allegedly screaming and threatening police officers and attempting to cross the lines of a crime scene. One report said he may have been intoxicated. He was charged with obstructing governmental administration, issued a desk-appearance ticket, and released yesterday afternoon, Mr. Kelly said.

Sources at the United Nations, who spoke on condition of anonymity, told The New York Sun that Mr. Van Schijndel, who lives in the East 50s, is a photo analyst working with Unmovic, the United Nations Monitoring, Verification and Inspection Commission, the agency responsible for inspecting Iraq for weapons of mass destruction. He could not be immediately reached yesterday for comment.

Despite the late hour, the grainy video footage from 17 surveillance cameras taken at 15 different locations on Third Avenue, between 51st and 52nd streets, showed that the U.N. analyst was not the only person in the area at the time of the blasts, Mr. Kelly said.

A taxicab was seen by police investigators passing by, as were a man riding his bicycle, a female jogger, and other pedestrians. At a press conference, the commissioner drew a laugh when he said, “This is New York.”

The initial speculation that the British Consulate was the target of the blasts were based on the election, the British role in Iraq, and age-old conflict over Britain’s rule of Northern Ireland.

The protests against Caterpillar gained momentum among activists in March 2003, after a 23-year-old college student, Rachel Corrie, was killed by a bulldozer in the Gaza Strip while protesting the razing of a Palestinian Arab home.

A spokesman for the Conference Board, Kenneth Goldstein, expressed doubts yesterday that a group of activists protesting Caterpillar or Ms. Fosler’s role as a board member at the worldwide company could have been responsible for the grenade explosions.

“Is it possible? Of course it’s possible, anything is possible, but it’s unlikely,” Mr. Goldstein said. He said Ms. Fosler could not be reached because she is traveling abroad.

“There could be no more than 1,000 theories behind this,” Mr. Goldstein said. “Caterpillar is only one of those theories – but a highly improbable one.”

A representative of Jews Against the Occupation did not return calls yesterday.

Another local organizer of last month’s rally, Patrick O’Connor, who is allied with a different activist group, the Stop Caterpillar Coalition, declined to comment on the grenade explosions.

Other organizers with the Stop Caterpillar Coalition, which is based in Chicago and held “an international day of action” April 13, said they were unaware of the grenade explosions in New York.

“I doubt very strongly that this has anything to do with the Stop Caterpillar movement,” one of the group’s organizers, Hatem Abudayyeh, said.

A representative of another activist group allied with the protests against Caterpillar, Liat Weingart, of Jewish Voice for Peace, said that the demonstration had been peaceful and that the groups organizing the rally were pacifist in nature and dedicated toward anti-violent protests.

“Any insinuation here is inaccurate,” she said.

After a preliminary investigation yesterday, Mr. Kelly said the pair of detonated grenades – one was of the typical pineapple shape, the other was short and smooth – had exploded in concrete flower planters outside the building. The grenades had been stuffed with black powder and topped with makeshift fuses similar to those found in fireworks such as “cherry bombs and M-80’s,” the police commissioner said.

A senior special agent with the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, Joseph Green, said the grenades had been manufactured for the military and had been used before. Inside, Mr. Green said, the makeshift grenades had been repacked with a generic form of black powder that can be purchased in gun stores across the country.

In the past five years, according to the federal agency, more than 100 such makeshift grenades have been repacked with black powder and detonated across the country. In 2000, a similar explosion took place when a military ammunition can packed with black powder exploded outside a Wall Street bank and shattered glass windows at two office buildings.

“This is something you can learn how to make on the Internet,” Mr. Green said of the devices.

Footage from surveillance cameras suggests that one of the makeshift grenades could have been lobbed at the building from across the street, Mr. Kelly said, while the other could have been stuffed in one of the planters, which are placed outside many Manhattan buildings by security officials to deter truck and car bombs.

The impact of the explosion inside the planter was strong: A 1-foot chunk of concrete was blown off. The front window of the building’s lobby shattered. Parts of the fuse were discovered stuck in a piece of metal on the ninth floor of an office building across the street.

Mr. Kelly urged anyone who may have witnessed any elements of the explosions to report any information to the authorities.