A Federal Bureau of Investigation inquiry was under way into the circumstances surrounding the death of Gary Mayers, 34, who was killed by a single shot to the chest on Thursday.
Mr Mayers, who died at the scene, was deported from the US a month ago. According to the immigration service yesterday, he was challenged by the patrol officer after crossing a remote stretch of the border illegally. He was shot after a violent struggle in the officer's car during which Mr Mayers allegedly tried to grab his gun.
Bill Strassberger, a spokesman for the immigration service, said the officer had gone to investigate a sighting of a backpacker jumping a border fence and then hitch-hiking along the main road. He said that Mr Mayers lunged at the officer as he radioed for help from his patrol car.
"The agent went to call for assistance and the individual violently attacked him and got on top of him in the front seat of his vehicle," Mr Strassberger said. "I believe the assailant was at least 50lbs heavier than the agent and there was a very tough struggle.
"At one point the attacker had the advantage and he pinned the agent down and had his hand on the weapon. He couldn't get the weapon out, so he then tried to choke the agent with the radio wire."
Mr Strassberger said the officer managed to break free and drew his gun. "He told his attacker to stop because he was under arrest, but he lunged again, and so the officer had no choice but to fire a round."
"The shooting occurred along the banks of the Kettle river, which runs from the mountains of British Columbia down to the Grand Coulee Dam. It is a sparsely populated area about 100 miles from Spokane, the nearest big town.
"Mr Mayers jumped a fence near the Laurier border crossing, in north- east Washington State, shortly before 10.15am local time. The confrontation took place five miles south of the border."
Mr Strassberger said that Mr Mayers acted suspiciously when the agent questioned him, so the officer returned to his car to radio for back-up.
Mr Strassberger could not say why Mr Mayers had been deported from the US. Nor would he speculate as to why he might have wanted to return to the country. Investigators making an initial survey of the scene said they found nothing suspicious in his backpack.
Border shootings of this type are highly unusual. The FBI said that it was looking closely into the circumstances surrounding it. The patrolman involved has been put on leave with full pay, as is customary in such cases.
The FBI is involved in the case because the patrolman is a federal officer.
A spokesman for the Foreign Office in London said that Mr Mayers' family would be contacted. He said any investigation into the matter would be left to the American authorities. "It is for the authorities there to investigate. We will be trying to contact the family and offer assistance," he said.
The spokesman said the British consul in Seattle would be handling matters in the US.