Ian Blair talks to the press in London, Thursday

London Police Commissioner Ian Blair says authorities investigating Thursday's attacks in the British capital have uncovered forensic evidence they consider a significant breakthrough.

He indicated that some of the devices found on three London subway trains and a bus remained unexploded but gave no further details. The commissioner noted similarities between Thursday's attacks and those two weeks ago, but said it is still too early to say if there is a connection between the incidents.

Earlier, British Prime Minister Tony Blair called the attacks an attempt to frighten Londoners but urged citizens to return to business as normal.

The noontime attacks come exactly two weeks after a series of blasts on subway trains and a city bus killed at least 56 people. Thursday's attacks forced suspension of service on at least three subway lines.

Two arrests were reported in the hours after the incidents, but Commissioner Blair said they were not related to the attacks.