LONDON - At least 40 people were killed in the explosions that ripped through three London subway trains and a bus on Thursday, according to a U.S. law enforcement official who spoke on condition of anonymity because British officials have yet to make public the death toll.

The near simultaneous explosions caused more than 300 injuries in what Prime Minister Tony Blair said was a "barbaric" terrorist attack.

U.S. authorities learned of the fatalities from their British counterparts, according to the official.

The explosions came a day after London was awarded the 2012 Olympics and as the Group of Eight summit was getting under way in Scotland.

The blasts went off between 8:30 and 9:30 a.m. local time. Implementing an emergency plan, authorities immediately shut down the subway and bus lines that log 8.4 million passenger trips every weekday. It brought the city’s transportation system to a standstill and left many central London streets deserted.

London police chief Sir Ian Blair said authorities had not received any warning. "We have been at a very high state of alert. Of course if there had been any kind of specific warnings we would have dealt with it," he said.

Four explosions confirmed
By early afternoon, London hospitals reported one death and said they had treated more than 300 people. The Royal London Hospital in the city's East End, which received the bulk of the injured, said 123 of about 180 casualties had been treated and released.

A senior police official said about 150 people were seriously injured. Police sources have said at least two people died.

British Home Secretary Charles Clarke said there were three explosions on the subway system and one on a bus. Earlier reports indicated six explosions.

“We do not know who or what organizations are responsible for these terrible criminal acts,” Clarke said.

Prime Minister Blair later left the G-8 summit for the day to return to London, while the other leaders continued the meeting.

"They will never succeed in destroying what we hold dear in this country and in other civilized nations throughout the world," Blair said before leaving and flanked by fellow G-8 leaders, including President Bush. "We shall prevail and they shall not."

Bush later warned Americans to be "extra vigilant" as they head to work.

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