Police announced Saturday that the man officers shot dead on a London subway car Friday morning was "not connected" with the bombing incidents of the day before.

"We are now satisfied that he was not connected with the incidents of Thursday 21st July 2005," police said in a statement.

"For somebody to lose their life in such circumstances is a tragedy and one that the Metropolitan Police Service regrets," the police said.

"The man emerged from a block of flats in the Stockwell area that were under police surveillance as part of the investigation into the incidents on Thursday 21st July," the statement said.

"He was then followed by surveillance officers to the underground station. His clothing and behavior added to their suspicions," the statement said.

It added that the circumstances that led to the man's death were being investigated.

Plainclothes police chased a South Asian man into the crowded subway car Friday morning and shot him in front of terrified passengers, as the hunt intensified for four suspects believed to have carried out abortive bomb attacks on the transit system the day before.

Police released security camera photos of the four and pleaded for witnesses to come forward. The images caught the suspects moments before and after bombs misfired in three subway cars and a bus in an apparent attempt to replicate the suicide bombings of two weeks ago that killed at least 56 people.

Witnesses said policemen with handguns caught up to the Asian man on a subway car at Stockwell station in south London. One of the officers shot him five times at close range, setting off a panicked evacuation of the car, according to passengers on the train.

"He looked like a cornered rabbit, a cornered fox, absolutely petrified," said Mark Whitby, one of the witnesses. "They pushed him onto the floor, bundled on top of him and unloaded five shots into him. I saw it. He's dead."

On Friday, Commissioner Ian Blair of the Metropolitan Police said the man, who was not identified, was "linked to yesterday's incidents" and had failed to obey police instructions to halt. But Muslim community leaders expressed fears that police were instituting a "shoot to kill" policy that could target innocent people because of ethnicity or appearance.

Police officials told British reporters Friday that the officers feared that the man, who was wearing a thick coat despite warm weather, had explosives concealed on his body and was about to detonate them amid the crowd in the station. They acknowledged that they found no bomb on the body.

Police believe those responsible for Thursday's small explosions are linked to the July 7 attacks -- in which the bombers were among those killed -- although they have yet to determine how, according to a British official with knowledge of the investigation. One person was slightly injured in Thursday's attacks.

Police announced Friday evening that they had arrested a man in south London as a terrorism suspect. They announced Saturday that a second man had been arrested as well but provided no details, wire services reported.

The arrests took place close to the site of one of the failed bomb attacks, and took place hours after police chased and shot dead another man in front of shocked passengers in the packed Stockwell Underground station.

Heavily armed police conducting the fast-moving dragnet swept through successive neighborhoods during the day, disrupting traffic, closing subway stations and contributing to the general sense of alarm and unease.

Officials said the Friday shooting incident, which occurred about 10 a.m., began when the man emerged from a residence that was under surveillance in connection with the bombing investigation. He spotted the plainclothesmen who were following him and fled into the nearby Stockwell subway station, one stop from the Oval station that was the scene of one of Thursday's explosions.

The man, who was wearing a baseball cap and thick coat, vaulted over ticket barriers with police in hot pursuit. "There were at least 20 of them and they were carrying big black guns," said Chris Wells, a company manager who was there. "Everyone was shouting, 'Get out! Get out!' " The man made it to the platform of the Northern Line and into a rail car before the police caught up to him. The shooting caused passengers to stampede out of the station or onto an adjoining train.

"Everyone was screaming and running off the Northern Line onto the Victoria Line platform, but I went the wrong way," said Sarah Simpkin, 27. "I banged on the doors of the Victoria Line train and the driver opened the doors and told everyone to get in. Then we got onto the train and got down low on the floor. There was a guy trying to calm us down."

"There was blind panic, with people shouting and screaming and just running away," said Chris Martin, who was on the platform when the shooting took place.

Later in the day, police clad in body armor and wielding automatic weapons triggered another wave of panic when they sealed off part of the Kilburn section of west London and raided a house. Witnesses described hearing five loud bangs that they thought were gunshots but turned out to be gas fired by police through the windows of the targeted house, whose door was smashed open by a battering ram.

Yassim Egal, 22, said he saw two women apprehended on nearby Harrow Road. "I was shocked by the way they arrested them," he said. "One of them had a shopping bag and a baby. They were very rough with them, shouting, 'Get against the wall.' They were telling everyone else to clear the area. It was really terrible."

The women were later released and police officials gave no explanation for the raid. Witnesses said one of the women was later carried away on a stretcher and taken to an ambulance.

Blair, the police chief, called the investigation "the greatest operational challenge ever faced by the Metropolitan Police Service." His officers, he said at a briefing, "are facing previously unknown threats and great danger. We need the understanding of all communities and the cooperation of all communities. We need calm."

But Muslim representatives expressed concern that the shooting marked a change of policy in the use of lethal force. "We understand there might have been reasons to do this, but we need to know in what context this man was shot and if it's true he was shot five times," said Muhammad Abdul Bari, deputy secretary of the Muslim Council of Britain, a coalition of prominent mainstream organizations. "Normally in this country this doesn't happen."

Muslims from South Asia have been especially anxious since three of the four men who allegedly carried out the deadly bombings of July 7 were identified as British-born Muslims of Pakistani descent. Officials from the council said Muslims had been calling in all day asking for details of the shooting and worrying that they could be singled out by police.

The four men depicted in security camera photos from Thursday all appeared to be black or South Asian. One British official said at least one of them is North African and others are believed to be from there.

One man wore a dark baseball cap and was carrying a backpack at the Westbourne Park subway station at 12:21 p.m. Police said he boarded a Hammersmith and City Line train at the station and attempted to detonate explosives in the bag as his rail car pulled into nearby Shepherd's Bush.

A man in khaki pants and a dark pullover with "New York" written on it was shown fleeing from the Oval station at 12:34; another image showed a bearded man in a dark blue shirt fleeing the Warren Street station minutes later.

The last picture, taken at 12:53, showed a middle-aged man with a mustache, wearing a white baseball cap and a T-shirt with a palm tree on the front, in the upper deck of a bus minutes before a backpack detonated.

Andy Hayman, assistant police commissioner for specialist operations, pleaded with the public to help track down the suspects.

"Do you recognize any of these men?" Hayman asked. "Did you see them at the three underground stations or on the bus? Did you see them at different locations? Did you see these men together, before or after the incident? Did you see them with anyone else?"

Thursday's attacks echoed those of two weeks ago in targeting three subway trains and a bus, almost simultaneously. But this time the bombs failed to detonate fully and the assailants survived and fled the four scenes. They left behind a vast amount of forensic evidence that police hope will lead to their capture and help unravel the plot behind both sets of attacks, which they suspect are connected to the al Qaeda network.

Officials said the explosives in both sets of attacks were the same type, probably made from triacetone triperoxide, a readily available but highly volatile substance. But the materials in Thursday's incidents were considerably smaller in quantity than the bombs used July 7. The authorities did not explain why the detonators inside each backpack failed to set off the explosives. At least one of the bombs contained nails and other shrapnel, according to British press reports, unlike the July 7 bombs.

"We believe these two attacks are quite probably connected, but we don't know how," said a British official who insisted upon anonymity. "We are convinced it's not a copycat attack."

The Abu Hafs al-Masri Brigade, an al Qaeda-linked group that asserted responsibility for the July 7 bombings, said in a statement on a radical Islamic Web site that it was behind the attacks, the Associated Press reported. But security experts questioned the authenticity of the claim.

Fred Barbash reported from Washington Staff writer Robin Wright in Washington contributed to this report.