Police officers check a van on London's Euston Road as security is heightened a day after the city was attacked by terrorists in a series of explosions on underground trains and a double decker bus

British police are conducting a massive investigation of Thursday's terrorist bombings in London that killed at least 50 people and left 700 wounded. Meanwhile, most of London's public transport resumed normal services Friday.

Police are on a grim search for clues in three subway tunnels and around a bombed-out bus in central London. They are trying to determine who carried Thursday's bombings, what explosives they used, and how they did it, all in hopes of catching the perpetrators.

British Home Secretary Charles Clarke says he cannot confirm if there was one or more suicide bombers involved. He says police have several theories, and are getting assistance from British and foreign intelligence services.

Mr. Clarke told British television that despite past police success in breaking up suspected terrorist plots, this one eluded the authorities. "Well the fact is we have very effective intelligence services but we did not predict this particular act at this particular time. And the reason for that is that we are always looking for a very small number of evil needles in a very, very large haystack, which is the city of London," he said.

Meanwhile, hundreds of thousands of London commuters got back on board buses and subway trains for the Friday morning rush hour. There was a mood of nervousness but determination among most commuters to get on with their lives and work, as London banker Matt Chapman explained. "It just makes Londoners more determined to show people that there is no fear and we are not afraid by things," he said.

Most of the transport system was operating normally, though the bombing sites remain sealed off crime scenes.