Forensic analysis should tell investigators whether this week's failed attacks were linked to the far more serious bombings two weeks ago, an anti-terror expert said Friday.

The four bombs detonated only partially Thursday and contained homemade explosives, Assistant Police Commissioner Andy Hayman said. He did not say whether they appeared similar to the bombs in the July 7 explosions that killed 56 people, including four bombers.

Metropolitan Police Commissioner Ian Blair (search) said authorities were conducting "a very, very fast-moving investigation" into the latest attacks. Police made two anti-terror arrests Friday and detectives were searching several London homes.

On Friday, police shot a man to death in front of stunned commuters in a south London subway station. Blair said the shooting was directly linked to the anti-terrorism investigations, but gave no further details.

Hayman said police were searching for four men believed to be linked to Thursday's explosions on three subway stations and a bus. He released closed-circuit television images of the suspects — including one running through a subway station and another hurrying off the upper deck of the targeted No. 26 bus — and urged anyone who spotted the men to call authorities.

"Do you recognize any of these men? Did you see them at the three Underground stations or on the bus?" Hayman asked in a televised appeal. "Did you see them at a different location? Did you see these men together? Or ... did you see them with anyone else?"

Some witnesses had reported seeing men they believed to be the bombers running away after Thursday's explosions.

Blair and Hayman did not say whether they believed the latest attacks were linked to the deadly July 7 bombings, which also hit three subways and a bus.

A single terrorist mastermind or bombmaker may have been behind both attacks, but investigators won't know until they compare the remains of the bombs from this week's attempt with explosive residues found after the July 7 attacks, said Alex Standish, editor of Jane's Intelligence Digest (search).

He warned there may be more cells at large, waiting to strike.

"If there's a common thread between all of them, then the likelihood is we're dealing with a far wider network of terrorism, possibly with a common mastermind," Standish said. "That may mean there is a third, fourth, fifth cell."

Standish said it was unlikely the two groups were in contact, saying they may have been part of a "molecular" structure in which separate groups take orders from the same leader but are unaware of one another.

Police have so thoroughly scrutinized the July 7 suspects' contacts — examining e-mail and mobile phone records and interviewing their acquaintances and witnesses to the attacks — that had the two groups been in touch, investigators would almost certainly have been aware of the latest attackers, Standish said.

Thursday's attackers also could have been copycats who modeled their attempt on the earlier bombers but lacked their technical expertise.

The second set of attacks clearly echoed the first — both groups struck on a Thursday and hit three subways and a bus almost simultaneously.

The latest attempts made "clear reference to what happened on the seventh. ... There's an obvious symbolism there," Standish said.

While he said it was too soon to rule out the copycat theory, if analysis of the explosives shows a link to the July 7 bombers, then "we have to assume this was as serious an attempt as the first attack."

It was not clear why the latest bombs failed to go off. Standish said that if the new bombers were linked to the earlier attackers, all may have used the same batch of explosives, which could have deteriorated since July 7.

That could suggest a single bombmaker equipped the two cells — and perhaps others — and then left the country.

Hundreds of officers are on the cases and the anti-terrorist squad is working around the clock, Blair said. Hayman said detectives were analyzing closed-circuit TV footage, examining witness statements and making door-to-door inquiries, all of which he described as "fruitful."

If the attacks are linked, evidence from the latest ones could prove crucial. Because more remains of Thursday's bombs, analysis of the explosives and any fingerprints or DNA (search) left behind could be easier to glean than in the wreckage of the July 7 explosions.