British authorities say the three bomb attacks on London's subway system Thursday came within seconds of each other, making it likely they were set off by timing devices, not suicide bombers.
Officials said examination of new technical data has led them to revise their earlier belief that the attacks were spread over nearly a half hour. A fourth bomb went off almost an hour later on a London bus.
A senior police official said the bombs were made of high explosives, suggesting that they were not home made.
About 50 people are known to have been killed, but the number is expected to rise. Authorities say they have not yet identified any of the dead.
Recovery crews are working in one of London's deepest subway tunnels where bodies are still trapped in wreckage. Officials say they expect recovery efforts to possibly continue into tomorrow-Sunday, in the hot, narrow, dark tunnel.
British officials say the attacks bear the hallmarks of al-Qaida, but there is no evidence directly linking the bombings to the terrorist network. There have been no arrests.
Information for this report is provided by AP, AFP and Reuters.