Charles Clarke said Tuesday he thinks it would be "very surprising" if the two incidents were not linked in some way, but no direct link has been discovered.

He says investigators also are trying to determine if the two groups of attackers received outside support or training, and if they are part of a wider international network.

On Monday, Mr. Clarke expressed concerned about the possibility of another terrorist attack in London.

Fifty-six people, including four suicide bombers, were killed in the July seventh attacks on three subway trains and a bus. Four other people tried but failed to set off more bombs on London's transport system two weeks later. All four are in police custody.