LONDON—British police killed one man and arrested another Friday as they mounted a massive manhunt for four men suspected of trying to plant bombs aboard three subway trains and a bus Thursday.
Sir Ian Blair, the chief of London's Metropolitan Police, called the investigation "the greatest operational challenge ever faced by" London police.
Police said they hadn't yet made a connection between Thursday's failed bombings and the blasts that killed at least 56 people on London trains and a bus two weeks ago. But they said the failed blasts had left them with a gold mine of evidence, from types of explosives to possible fingerprints of the bomb makers, to compare with chemical residue from the earlier blast sites as well as bomb-making materials recovered in a series of raids in the cities of Leeds and Luton.
Police also were able to find images of the bombing suspects quickly on closed-circuit television tapes and make them public, urging people to phone in any information about the men's identities.
The day's most dramatic moments came at the Stockwell subway station, where police chased a man onto a subway train, then, fearful that he might detonate a bomb in the crowded car, shot him dead as he fell to the floor.
Police didn't identify the man and said they didn't know if he was one of Thursday's bombing suspects. But they said he'd been spotted coming from a house that was being watched as part of the investigation into Thursday's events and was "followed by surveillance officers to the station."
"His clothing and his behavior at the station added to their suspicions," a police statement said.
Witnesses said the man was wearing a winter coat and carrying a backpack. He reportedly jumped a turnstile and began running. Police chased him. When he dashed onto a train, he stumbled and may have fallen, as police fired five to seven times.
Gillian Breen, who works at a fruit and flower stand near the station, said she saw about a dozen officers in plainclothes charge into the station at around 10 a.m. A few seconds later passengers came streaming into the streets.
"We just saw police running into the station with guns, telling everybody to move," she said. "It's quite a traumatic experience."
Blair declined to comment on whether police found explosives on the man, but said the shooting "is directly linked to the ongoing and expanding anti-terrorist operation."
"Any death is deeply regrettable," Blair said. "I understand the man was challenged and refused to obey."
Police revealed no details about the man who was arrested, except to describe him as involved in Thursday's attacks. They said he was arrested near the Stockwell station.
The police clearly were aided in their investigation by the speed with which they were able to find images of the would-be bombers on security camera tapes. Because the bombs failed to go off and people immediately dashed off the trains, police knew the precise time it had happened and which sections of tape to search. They also credited the tapes with prompting their surveillance of the nearby house that sparked Friday's shooting.
Unlike the photos released after the July 7 attacks, in which the suspected bombers looked relaxed as they moved toward the targets, the four photos released Friday showed nervous men, after the bombs have failed. In two, men are running from the stations. In all, the men look lost and confused about what to do next.
The suspected failed bombers apparently tried to change their appearances after fleeing. Police said a man who fled the Oval Street station wore a "New York" sweatshirt, and such a shirt was later found in Brixton, an area of London sometimes connected to radical Islam.
The suspected bomber pictured running from Shepherd's Bush was wearing a dark shirt; witnesses told police he later put on a white vest.
Police also released a photo of one of the bombs, a backpack surrounded by what's described as explosive material that spilled out of it during the partial explosion.
Blair said police hadn't yet determined why the bombs didn't explode. News media speculation has suggested the explosive materials were mixed incorrectly or may have deteriorated.
Assistant Commissioner Andy Hayman said the bombs appeared to have been homemade and were in dark bags or backpacks.
He made it clear that police think that connections between Thursday's attempted bombings and the July 7 blasts go beyond the fact that each targeted three subway trains and a bus.
"There are other features that are emerging which are also of interest to detectives," he said.
(Potts is a Knight Ridder Newspapers special correspondent.)
(c) 2005, Knight Ridder/Tribune Information Services.
PHOTOS (from KRT Photo Service, 202-383-6099): ATTACKS-LONDON
GRAPHIC (from KRT Graphics, 202-383-6064): 20050722 ATTACKS shot
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