The head of London's (search) police force expressed deep regret Sunday for the slaying of a Brazilian man by officers who mistook him for a suspect in the recent terror bombings, but defended the police use of deadly force.

Metropolitan Police Commissioner Sir Ian Blair (search) also said there were similarities between the explosives used in Thursday's failed bomb attacks and those detonated in the deadly July 7 bombings. But he said investigators still had no proof the two strikes were linked.

"The equipment in the bombs had all the elements that it should have but it didn't work," Blair told Sky News TV, referring to the explosives that failed to detonate properly Thursday on three subway cars and a double-decker bus.

"It had some similarities" to the devices used by four homicide bombers who killed themselves and 52 other people in three subway trains and a double-decker bus July 7.

Later Sunday, relatives and friends of people killed in those explosions planned to visit the sites of the attacks after a police briefing on the state of the investigation.

Commissioner Blair, asked if the latest attacks and the July 7 bombings were connected, replied, "We have no proof that they are linked but clearly there is a pattern here."

Police have made two arrests after Thursday's botched attacks. Officers have not released the identities of the arrested, nor many other details about them. Blair said the arrested remained in custody, but added that officers were "still anxious for any sighting of the four individuals" who carried out the Thursday strikes. Closed-circuit TV stills of the suspects were made public last week.

Police also said a package found in west London on Saturday may be linked to the devices used in Thursday's bomb attempts, but gave no details of what the package contained.

The man shot Friday at the Stockwell subway station was identified as Jean Charles de Menezes (search), 27, a Brazilian citizen. Witnesses said he was wearing a heavy padded coat when plainclothes police chased him into a subway car, pinned him to the ground and shot him five times in the head and torso in front of horrified passengers.

Police initially said they believed Menezes was linked to the investigation of Thursday's attacks, but later said he had no connection to the bomb attempts.

"This is a tragedy," Commissioner Blair said of the shooting. "The Metropolitan Police accepts full responsibility for this. To the family I can only express my deep regrets."

He defended the officers' shooting to kill, saying such action only applied when lives were believed to be at risk.

"I am very aware that minority communities are talking about a shoot to kill policy; it's only a shoot to kill in order to protect policy," he said.

British police had drawn from the experience of other countries that have had to deal with suicide attackers, he said.

"It is drawn from experience from other countries, including Sri Lanka. The only way to deal with this is to shoot to the head," Blair said. "There is no point in shooting at someone's chest because that is where the bomb is likely to be," Blair said.

Blair spoke of the problem his officers faced.

"What we have got to recognize is that people are taking incredibly difficult fast-time decisions in life threatening situations," he said. "... What's most important to recognize is that it's still happening out there. There are still officers out there having to make those calls as we speak."

Police said Menezes attracted police attention because he left a building that was under surveillance after Thursday's attacks. They said he was then followed by surveillance officers to the station, and that his clothing and his behavior at the station added to their suspicions.

Brazil's Foreign Ministry said it was "shocked and perplexed," over the death of Menezes, whom they did not name, but described as "apparently the victim of a lamentable mistake."

Britain's Foreign Office said that Brazilian Foreign Minister Celso Amorim, who is visiting London, would hold a private meeting with a British official to discuss the circumstances of the shooting.

Menezes was an electrician who had been working in Britain for three years, said his cousin Alex Pereira, who also lives in London.

"He was a 100 percent good guy who never did anything wrong and had no reason to run," Pereira said. "I don't think he ran from police. I don't think he would do that. They can't show anything that shows that he had."

The shooting was an indication of the anxiety in the city of about 8 million people. A police watchdog organization, the Independent Police Complaints Commission, said it would investigate the shooting but make sure not to hinder the bombings probe.

Shami Chakrabarti, director of the civil rights group Liberty, said such an investigation was critical for reassuring the public.

"It's incredibly important that society remains united at such a tense time, it's very important that young Asian men don't feel that there is some kind of trigger-happy culture out there," Chakrabarti said.

Sir Iqbal Sacranie, secretary-general of the Muslim Council of Britain, said, "It's absolutely vital that the utmost care is taken to ensure that innocent people are not killed due to overzealousness."