A British counterterrorism official, Peter Clarke, said Thursday Shahzad Tanweer was responsible for the blast on one of three subway trains, while Hasib Hussain carried out the attack on a double-decker bus.

The men were two of three British-born suspects with Pakistani ancestry. British media report the fourth was a Jamaican-born Briton.

The announcement came as authorities hunt for co-conspirators who they believe helped the bombers plan the attacks that killed at least 53 people.

Police say one suspect, a British-born man of Pakistani descent with possible links to al-Qaida, is believed to have masterminded the bombings.

Police continue to hold a relative of one of the suspects arrested Tuesday.

Meanwhile, London came to a virtual halt Thursday as vehicles stopped and people gathered in public squares for a moment of silence in remembrance of the victims.