LONDON — Explosive devices detonated harmlessly yesterday on three London trains and a double-decker bus in an eerie reenactment of the July 7 transit attacks that killed 56 persons exactly two weeks earlier.
The failed attacks sent passengers running in panic and shut down subway trains, bringing the capital to a halt for the second time this month, but failed to inflict the death and destruction of the earlier attacks.
One person was initially reported injured but authorities said later the only casualty had suffered from an asthma attack, the Associated Press reported.
It was not clear last night whether the perpetrators were linked to the July 7 bombers or were carrying out some sort of bizarre copycat action.
However, Metropolitan Police Commissioner Ian Blair said the object of yesterday’s attacks had been to kill, and that he thought there was a “congruence” between the two operations.
Two persons were detained during the day and were being questioned last night, one of whom was arrested near the prime minister’s 10 Downing Street residence. The other was arrested near the Warren Street subway station, the site of one of the attacks.
Passengers gave chase to one of the attackers after his backpack exploded but failed to catch him, a witness told Sky News television.
“There was a little explosion. As soon as the door opened, the man ran away and people were trying to run after him. There were three men struggling with him but he ran off and they couldn’t catch him,” she said.
Security sources said police had found pictures of all four bombers on closed-circuit television cameras, and were considering releasing the images to the public.
Police also appealed to the public to e-mail or text-message any pictures they had taken on videos, cameras or mobile phones.
Mr. Blair, the police commissioner, said authorities were examining unexploded material found at the scenes of the bombing attempts, which like the July 7 attacks were spaced out to the north, south, east and west in what some described as the shape of a burning cross.
The earlier bombs ripped apart rail cars and a double-decker bus, killing 56 persons and injuring hundreds more. Yesterday’s efforts caused a noise likened by one witness to a champagne cork popping and created mostly smoke.
Some experts speculated that the bombs, intended to explode simultaneously, had been faultily assembled or the detonators had been incorrectly primed.
“One bomb-maker could have made the same mistake with all four bombs,” said security expert Crispin Black. “Or else material manufactured previously had lost its lethality.”
The July 7 bombs were reported to have been made using acetone peroxide, the same easily manufactured high explosive used by some Palestinian suicide bombers and by British would-be shoe bomber Richard Reid.
Acetone peroxide is known to degrade in long-term storage, making it unsuitable for conventional explosive ordnance.
British police were hopeful last night that the latest attacks may provide clues that will lead them to the planners and suppliers behind the earlier blasts. “It presents forensic opportunities,” said Mr. Blair.
Security sources said an early sampling indicated that the explosives closely matched those used two weeks ago. A backpack found smoking and split but otherwise undamaged on the upper deck of the bus may give investigators key clues including DNA or fingerprints, they added.
The July 7 attackers also carried their bombs in backpacks.
Police also are hopeful of capturing the perpetrators of the latest attacks, with the prospect of obtaining information about who directed them. The July 7 attackers all died from the bombs they detonated.
The London transport system managed to keep most trains and buses running despite the incidents, but tourists said they were becoming too alarmed to stay in a city under apparent siege.
“We ‘re going home tomorrow,” a pair of German tourists told a television station. “It’s getting too much.”
Jerry Seper in Washington contributed to this article.