Authorities in London say the death toll from Thursday's bus and subway bombings now exceeds 50. Britain's Home Secretary Charles Clarke says investigators are taking very seriously a claim of responsibility by a previously unknown group calling itself "The Secret Organization of al-Qaida in Europe."

Meanwhile, explosives experts and forensic teams are examining the scenes of the blasts.

Most underground transit was operating for commuters Friday morning

Londoners who ventured back to the city's subway and bus system for their morning commute had to deal with numerous delays. The situation following Thursday's explosions was complicated by at least two security alerts. Police briefly evacuated passengers at Euston Station, allowing passengers back into the station ten minutes later. Officers also cordoned off an area outside the Liverpool Street Station, after a suspicious package was found at a nearby restaurant.

Meanwhile, London's Metropolitan Police said the bombs used in the attacks held less than 4.5 kilograms of explosives, light enough to easily carry in a bag or knapsack. But police say they have found no evidence they were detonated by suicide bombers.

Andy Hayman is the assistant commissioner of the Metropolitan Police. "At this stage we do believe that each device put onto the tube trains was likely to be put on the floor of the carriage. In respect to the bus at Tavistock Square, it's likely it could have been on the floor as much as it could have been on the seats, so again, that's very much up in the air."

Police commissioner Sir Ian Blair said he agreed with the assessment of British Foreign Secretary Jack Straw, who said the nature of the attacks suggest the work of the al-Qaida network. But Mister Blair said it was too soon to draw any such conclusion.

Charles Clarke

Charles Clarke, Britain's Home Secretary, said British intelligence had no prior warnings. "We are obviously looking very, very carefully at all of our intelligence to see whether anything was missed but we don't believe anything was missed and it simply came out of the blue.

Many Londoners who refused to abandon their daily bus or subway commute kept the traditional British stiff upper lip.

"I think Londoners are a bit resilient really and commuters will carry on,” said one commuter.

"[I’m] working as normal. Carrying on. Not being beaten,” said another commuter. "I worked for Cantor Fitzgerald (a New York investment firm devastated at the World Trade Center) at the time of the 9/11 disaster and it all brought it back with what they went through. It was our turn yesterday."

Increased security was evident throughout the London public transit system. Police and transit officers took time to help commuters deal with alternate routes, even bringing a smile to someone perhaps too young to comprehend the anxiety throughout her city.