Police investigating failed July 21 bomb attacks in London (search) said Monday they had arrested two men during raids in the city, as authorities tried to determine whether there were links between that attack and the transit bombings two weeks earlier.

A total of 23 people have been arrested in connection with the failed bombing attempt, including the four main bombing suspects in police custody in London and Rome. British transport police, meanwhile, dispatched reinforcements from around the country Monday to patrol London's subway system in a show of force meant to discourage more attacks.

The July 21 bombing came exactly two weeks after July 7 attacks that killed 52 people plus all four suicide bombers. Both attacks both hit three subway cars and a red double-decker bus, but the July 21 attackers' explosives failed to detonate and took no lives.

The men in the arrests announced Monday have been detained "on suspicion of the commission, preparation or instigation of acts of terrorism," a Metropolitan Police (search) spokeswoman said. They were arrested after searches of three properties in the Stockwell and Clapham areas of south London.

"The searches are in connection with the ongoing investigations into the incidents on the London transport network on the 21st of July," the spokeswoman said.

With many Londoners fearful of a third round of attacks on the Underground (search) and bus systems, authorities looked for any solid links between the first two plots, which appeared similar on the surface.

"There is a resonance here," Metropolitan Police Commissioner Ian Blair noted hours after the second round of bombings.

It's likely the two cells — the first made up mostly of Pakistani Britons and the second of immigrants from East Africa — didn't know of one another but reported to the same organizer or bomb-making expert, said Alex Standish, editor of Jane's Intelligence Digest.

"That has to be the assumption (investigators) are working on at the moment," he said. "Only by uncovering the structure can they hope to discover whether there are further cells operating in the U.K."

"There is absolutely no reason why a third, fourth, fifth cell shouldn't exist," Standish said.
But, he added, interrogation of the captured July 21 suspects is unlikely to lead police to other cells, because terror networks are set up so that those who carry out separate attacks have no knowledge of one another.

Police have not said whether they think the two sets of bombers were part of the same network, reporting to a common supervisor.

News reports have said suspects from both groups worshipped at the Finsbury Park Mosque in north London, known as a hotbed for Islamic radicals. They may even have gone whitewater rafting in Wales weeks before the attacks.

Investigators have played down reports saying that they believed a third terror cell was poised to strike.

But "you can't rule out the possibility that there may well be other people with similar intentions," a police spokesman said, speaking on condition of anonymity because department policy does not allow him to be quoted by name.

Security in the Underground was heavy Monday, with officers in bright yellow jackets posted outside many stations. Simon Lubin, a British Transport Police spokesman, said the presence was higher than over the weekend, but not as strong as Thursday, when a huge force patrolled the transit system exactly three weeks after the first bombing attack and one week after the second.

In Italy, where one of the suspected July 21 attackers was being interrogated, Carlo De Stefano, head of anti-terror police, said the investigation so far indicated that Hamdi Issac was "part of a loosely knit group rather than a well-structured group."

Issac was charged in Italy on Monday with association with the aim of international terrorism and possessing false documents, said Antonietta Sonnessa, his lawyer. Italian police said his extradition to Britain wouldn't take long.

De Stefano did not say whether she thought Issac's cell had ties to the earlier bombers. Italian media have reported that Issac told police his group was not linked to either the July 7 bombers or the Al Qaeda network.

Sonnessa told Britain's ITV News that Issac claimed the July 21 plot was put together the day before, "in a meeting with this group of friends."

Tim Ripley, a researcher at the Center for Defense and International Security Studies in Henley, England, said that was doubtful. But he felt the two bombing groups operated independently, which he finds worrisome.

"At least if you had had a linkage, you've got something to go on," he said. "Whereas if you have no idea, you're looking for a needle in a haystack."

Forensic evidence will likely be crucial to police efforts to establish whether the groups had any ties.

News reports have suggested both used explosives made from acetone peroxide, also known as TATP, but police would not comment. TATP is relatively easy to produce.