The death toll from Thursday's bus and subway bombings in London now stands at more than 50.
British authorities confirmed that more than 700 people were wounded. The effort Friday was to study the blasts and find out more about the perpetrators.
In London, Friday, buses and some underground trains carried jittery commuters to and from work.
"There was a split second when we just thought, OK, that could happen, and you just feel afraid and you just don't feel right," said one Londoner.
Some people continued their search for loved ones not heard from since the attacks on the city's transportation system.
Prince Charles and his wife, the Duchess of Cornwall, visited a hospital where survivors injured in the blasts were being treated.
His mother, Queen Elizabeth II, did the same.
"Atrocities such as these simply reinforce our sense of community, our humanity, and our trust in the rule of law. That is the clear message from us all," she said.
Police examined the scenes of the blasts. They said the bombs were small enough to fit in a backpack or a bag, and that the bombs could have been placed on the floor or under the seats. London officials said they were taking seriously a claim of responsibility from an al-Qaida related group.
Professor Barnett Rubin from New York University, an expert on al-Qaida, agreed the attacks had the markings of the terrorist group. "Because al-Qaida uses simultaneous bomb blasts in different places, in very dramatic locations, in order to achieve the maximum of publicity effect," he said.
Another American expert defined al-Qaida as splinter groups without a central authority. Julianne Smith, a security expert at the Center for Strategic and International Studies in Washington, DC, said those responsible are likely a small group that identify with the overall mission of al-Qaida. "All indication right now shows that this is more or less a British-bred organization, Muslims who are residing inside the UK that are out to undertake grievances they have against the British government," she said.
London police have not ruled out the possibility the attacks may be the work of suicide bombers but they say they do not have evidence it.