Investigators hunting four fugitives wanted for the abortive bombing of the city's transit system last week have linked them to the four presumed suicide bombers who killed 52 bystanders and wounded 700 others two weeks earlier, a British official said Sunday.

Meanwhile, the country's highest-ranking police officer made a public apology for the killing of a Brazilian man at a subway station Friday by plainclothes officers who mistook him for a suspected terrorist.

Many of the Brazilian's relatives and friends rejected the official explanation for the shooting of Jean Charles de Menezes. The 27-year-old electrician was on his way to a job when he was chased into a subway car by undercover policemen, one of whom shot him in the head five times in front of horrified passengers.

An official confirmed press reports that investigators had found a brochure for a whitewater rafting company in the backpack that one of the fugitives had left on the upper deck of a double-decker bus in east London on Thursday. Earlier, police had determined that at least two of the men presumed to have been suicide bombers in the deadly July 7 attacks -- Mohammed Sidique Khan, 30, and Shehzad Tanweer, 22 -- had taken part in a rafting trip at the company's center in northern Wales in early June.

Khan and Tanweer appear smiling and relaxed on a raft in a photograph of participants from that day.

Police are investigating the possibility that all eight of the men met up that day, said the official, who spoke on condition of anonymity, and that they might have been brought together by another person who helped coordinate the two attacks and might provide a link to the al Qaeda network. The discovery of the brochure was first reported by British newspapers and the New York Times.

One working police theory is that the would-be bombers of July 21, whose backpacks of powerful homemade explosives failed to detonate, are from a cell of northern Africans living in the London area, the official said. The men who police believe carried out the July 7 attacks were British Muslims -- three of them of Pakistani origin from the northern England city of Leeds, the fourth a Jamaican-born convert to Islam.

The other potential link between the two groups is the explosives. Investigators believe triacetone triperoxide was used in both sets of attacks, although they have still not conclusively identified the substance in the July 7 bombings. Traces of it were found in the pipes of a tub in a Leeds apartment believed to have been rented by one of the bombers, as well as in nine small bombs found in a rental car left by one of the men in a train station north of London.

Police announced that they arrested an unidentified man Sunday evening. He is the third man being held for questioning under anti-terrorism laws since Thursday's failed bombings on three subway trains and a bus -- attacks that injured only one person but mirrored the deadly strike two weeks earlier. Bomb disposal experts carried out a controlled explosion of a suspicious package they said might be linked to the attack.

The Sunday Telegraph reported that Khan, believed to have been the leader of the London suicide team, met in Pakistan last fall with an alleged al Qaeda operative, Mohammed Yasin, alias Ustad Osama, an explosives specialist with the extremist Harkat-e-Jihad group. The newspaper cited unnamed Pakistani intelligence sources for its claim.

A veteran of militant training camps along the remote Afghanistan-Pakistan frontier, Yasin, who is in his thirties, is reputed to be an expert at manufacturing "suicide jackets," the newspaper reported. His name is included on a list of the 70 most wanted terrorists issued by Pakistani officials in December 2003.

While detectives hunted the would-be bombers, Metropolitan Police Commissioner Ian Blair offered his "deep regrets" to the de Menezes family for Friday's shooting but said his department would not change the "shoot to kill" policy it uses when facing what it deems a genuine terrorist threat.

"The procedures have been reviewed and reviewed and reviewed for many months," he told Sky News in an interview. "This is not a Metropolitan Police policy, it is a national police policy."

Blair said that when confronting suspected suicide bombers who might have explosives strapped to their chests, "there is no point in shooting somebody's chest -- the only way to deal with this is to shoot in the head." Police officers had to make "incredibly fast decisions under life-threatening conditions," he said, pledging a full investigation into the killing.

According to the official account, plainclothes officers who had received information tracing a suspected terrorist to an apartment block in south London mistakenly followed de Menezes when he emerged from one of the buildings on Friday morning. Police said he was wearing a baseball cap and a heavy jacket on a warm day, and they suspected he might be carrying explosives. When he got off a bus and entered the Stockwell subway station, they raced after him in hot pursuit and shot him dead in a subway car.

De Menezes's friends and relatives did not accept Blair's statement of regret. "Apologies are not enough," his cousin, Alex Alves Pereira, tearfully told reporters at his London apartment. "I believe my cousin's death was result of police incompetence."

Pereira said his cousin, who had lived in London for three years, was from the Brazilian city of Gonzaga. He retraced his cousin's steps Sunday afternoon with a pack of reporters and cameramen in tow, taking the same bus that de Menezes took when he was followed by police.

"If he had a bomb he could have blown it off on the bus," Pereira told reporters. "There's no explanation for what they have done."

Friends and relatives peacefully demonstrated in the rain outside police headquarters at New Scotland Yard, and Brazil's foreign minister, who was in London for a previously scheduled visit, met officials at the Foreign Office to seek an explanation for the shooting. "The Brazilian government and the public are shocked and perplexed that a peaceful and innocent person should have been killed," Celso Amorim told reporters. "Brazil is totally in solidarity with Britain in the fight against terror but people should be cautious to avoid the loss of innocent life."

At a Brazilian cafe on Oxford Street in central London where de Menezes was described as a regular, owner Luiz de Souza denounced the slaying as "barbaric" and said police owe the public a full explanation.

"Five shots in the back because he is supposed to have too big a coat on?" de Souza wondered. "He comes in here almost every day, he always wears the same jacket, I know this jacket. It is a Levi's jeans jacket."

"From what I heard, the police were in plainclothes and were following him. Maybe Jean was worried. Then they draw guns, and probably he got scared and ran away," de Souza said. "Everyone is scared these days."

De Souza dismissed the possibility that a language barrier might have played a role in the shooting. "Jean spoke very good English. Very good. Better than mine, almost, and I've been here 20 years. He would have understood every single word they said."

De Souza said he last saw de Menezes two days before he was killed, when they had lunch together at the cafe, where a large green, yellow and blue Brazilian flag shrouded in black gauze now hangs in the upstairs picture window.

"He was a very sociable person. Everyone here is very shocked, very sad and upset. Many are saying they're going back to Brazil because it's getting worse here day by day. I know many who already have gone back," de Souza said.

At the Stockwell station where the shooting occurred, there were more than 20 bouquets, most accompanied with angry messages. "Five Bullets in the Head -- Who's Guilty?" said one poster. "Their Game -- Our Blood" said another.

Staff writer Tamara Jones and special correspondent Glenda Cooper contributed to this report.

Maria Otone de Menezes, center, mother of Jean Charles de Menezes, is comforted by relatives in Gonzaga, Brazil. Family and friends of the slain man rejected the official explanation for his shooting.Jean Charles de Menezes, 27, was shot dead Friday in London.