London's police chief said investigators were gathering forensic evidence Thursday that could offer a "significant break" in determining who carried out the latest attack on the transit system.

Metropolitan Police Commissioner Ian Blair (search) did not give details of the evidence, but a terror expert said it could include DNA and fingerprints from explosive devices, as well as the origin of any detonators.

Blair said it was too early to determine whether the incidents were related to the July 7 attacks that killed 52 people and the four suspected bombers two weeks ago.

"There is a resonance here, isn't there, I mean these are four attacks, there were four before," Blair told reporters. "Whether or not this is directly connected in the sense of carried out by the same group of people, however loosely knit that is, I think that's going to take just a little bit longer."

Police said there were no reports of bomb blast injuries Thursday. Later in the day, they said two arrests had been made in connection with the case.

Two men were arrested — one near Prime Minister Tony Blair's (search) Downing Street residence, and the second, later released without charge, near Tottenham Court Road, which is close to the Warren Street Underground station.

London's police commissioner said police had sealed off the attack sites to look for clues. Authorities were no doubt already spooling through videotape from surveillance cameras that had provided critical information in determining the identities of the July 7 bombers.

"We do believe that this may represent — may represent — a significant breakthrough in the sense that there obviously is forensic material at these scenes that could be very helpful to us so I feel very positive about some of these developments," Blair said.

He also said he was not in a position to say whether the attacks were related to Al Qaeda (search). The suspected July 7 bombers were all Muslims.

"I think it's too early to say anything like that — just too early," Blair said. "But we can all see in front of us that the attacks have a similar pattern to the previous one."

Paul Rogers, a terror expert at Bradford University, said investigators would have lots of forensic evidence to work on and that the attacks could point to the existence of another terrorist cell in Britain.

"They will have the devices and much can be done to them in terms of fingerprinting, DNA, the origin of the detonators and where the bags were bought," he said.

"The one ominous thing is that this appears to be a group of a similar nature to the previous July 7 bombers," Rogers said. "It implies there might be another cell primed and ready to attack."

The attacks of two weeks ago have been linked to Islamic extremists, and Blair appealed for people not to lash out against Muslims in general.

"No community should be smeared with responsibility for these matters," he said. "These are criminal acts and we are in pursuit of a set of criminals in relation to it."

London Mayor Ken Livingstone appealed for broad help from city residents for tips that could help in tracking down the culprits.

"In particular I'd like to ask those who will be leading religious services over the next few days to take their congregations through these events, the immorality of these events, and do everything in their power to convince people ... to come forward," he said.