Search For Answers In London

(CBS/AP)  A string of rush-hour explosions tore into at least three London subway trains and a double-decker bus on Thursday in the worst

attack (video)

on London since World War II, killing at least 37 people and injuring 700. Britain's foreign secretary said it had the "hallmarks of al Qaeda."

Prime Minister

Tony Blair (video)

blamed Islamic terrorists, saying those behind the "barbaric" attacks had acted "in the name of Islam" but that he knew most Muslims worldwide "deplore this act of terrorism."

"They are trying to use the slaughter of innocent people to cow us," Blair said after returning briefly to London from the G-8 summit in Scotland. "They should know they will not succeed."

Foreign Secretary Jack Straw, still at Gleneagles, said, "It has the hallmarks of an al Qaeda related attack. ... There is an assumption, obviously, that this is an al Qaeda based organization. It has the hallmarks of an al Qaeda-based organization and also its ruthlessness." Police said there had been no warning and that the train blasts went off within 18 minutes, starting at 8:51 a.m. (3:51 a.m. EDT). The bus blast happened at 9:47 a.m. (4:47 a.m. EDT), they said.

CBS News Correspondent Jim Stewart reports British investigators were pouring over surveillance camera tape looking for clues, if not the outright faces of the terrorists who pulled off Thursday's attack. There are tens of thousands of surveillance cameras on London streets — 6,000 alone in the London subways.

"Clearly, we've had considerable success in the past using closed circuit television footage in order to trace the movements of the people involved," Deputy Assistant Commissioner Brian Paddick tells CBS News. "That will be one of our first priorities."

Early in the day, investigators believed the bomb that went off aboard the double-decker bus was the work of a suicide bomber, Stewart reports. But by late afternoon, they appeared to be leaning against that notion.

Stewart reports investigators now suspect all the bombers used what they called the "step-on, step-off" delivery system of dropping off a bag and quickly leaving.

Stewart added that British police thought they had a quick break in the case with the discovery of what looked like an unexploded terrorist bomb, but it turned out to be something else.

Investigators were also gathering remnants of the bombs hoping, as police did in Madrid and Bali, to discover key elements of the devices which might lead them to suspects. In Madrid, for example, the cell phones used as remote detonators, were purchased on the black market.

In London, police said they could confirm at least 37 people had been killed, including two in the bus attack. Three U.S. law enforcement officials told The Associated Press at least 40 were killed. French Interior Minister Nicolas Sarkozy later said the death toll had risen to 50, citing a conversation with his British counterpart, but that could not immediately be confirmed.

Police said at least 700 were wounded, many of whom emerged bleeding and dazed from the Underground. Among them, at least 45 were in serious or critical condition, including amputations, fractures and burns, hospital officials told The Associated Press. Two young women from Knoxville, Tenn., were among those treated for injuries in the Underground, said their father, Dudley Benton.

As Wednesday's jubilation at winning the Olympics gave way to the terrible shock of Thursday's attacks, a shaken Blair rushed back to the capital. He then delivered an almost Churchillian appeal for unity, saying in a televised address that it was "a very sad day for the British people, but we will hold true to the British way of life." He praised the "stoicism and resilience of the British people."

Blair arrived back in Gleneagles in Scotland late on Thursday after returning to London for crisis talks following the attacks.

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