Britain's most senior police officer said Friday that government investigators intuitively suspected that the groups responsible for two bomb attacks on the London transport system last month may have been linked, although he stopped short of saying that they had evidence establishing any direct connection.

"At the moment we have nothing that links the two groups, but the investigation still continues and there is an intuitive view that says that each group of four are so similar that we must have concerns, and we do," said Ian Blair, Metropolitan police commissioner, in an interview with the Evening Standard newspaper.

A key question remains whether the two groups of bombers acted together or perhaps under the direction of the same planner. Blair's comments were significant because police have cautioned against assumptions that the two groups were linked, despite similarities in the attacks. Each incident involved coordinated attacks on three subway trains and a double-decker bus by young men carrying explosives in backpacks.

The first attack, on July 7, killed 56 people, including the four presumed bombers, and wounded 700 others. The second attack, exactly two weeks later, resulted in no casualties because the explosives failed to detonate. British police have arrested four suspects, and a fifth is in custody in Rome, where he was arrested. Italian police said they would extradite the man to Britain in the coming weeks.

Blair, in the interview, warned against concluding that a single person could have engineered both attacks.

"The question is more around: Was there some training that was held in common? Is there some set of instructions somewhere? But these are things that the investigation will continue with," he said.

He also said police remained concerned that other attacks may be in the works. "Although we have not yet established any direct links," he said, "intuitively there are such similarities between the methodologies and the equipment that we must think there is a possibility of others."

Also Friday, the family of Jean Charles de Menezes, a Brazilian electrician mistakenly killed by British anti-terror police on the day after the failed attack, issued an emotional appeal demanding Blair's resignation.

"My family want the truth for the sake of my family, for the sake of the people of London," said de Menezes' cousin, Alessandro Pereira, in a statement read just outside the subway station where police shot Menezes seven times in the head. "In Jean's name I say that those responsible should resign. Ian Blair should resign."