London ? Four bombs that each contained less than 10 pounds of explosives and were small enough to be concealed and carried in backpacks caused the carnage in London’s mass transit system that killed at least 49 people and injured 700, police officials announced Friday.

More than one person planted the explosives, police said, and there was no evidence so far that the attacks – the worst in London since World War II – were suicide bombings. Ian Blair, head of the city’s Scotland Yard police force, warned that the perpetrators were probably still at large, and he urged public vigilance.

The official count of 49 did not include an unknown number of bodies still trapped in the wreckage of a subway car wedged 70 feet below ground in a tunnel near the Russell Square subway station. That attack caused the largest toll of death and devastation in the Thursday morning rush-hour bombings.

Senior officials confirmed that they had downgraded the security alert level in May and had no warning of the attacks. Charles Clarke, the Cabinet secretary in charge of domestic security, defended Britain’s intelligence services but conceded that there had been “a failure of intelligence in the sense that we didn’t know this was coming.”

As wary Londoners returned to their offices and shops Friday morning, emergency workers and investigators pored over charred and twisted wreckage at four bomb sites, searching for victims and clues to the identity of the terrorists.

Officials said no arrests had been made, and they publicly identified no suspects. But they said it was likely that the perpetrators were associated with the al-Qaida network. “The number one task today is to track these people down,” Clarke told reporters.

One working theory, said an official with knowledge of the investigation, was that the bombers had originated from King’s Cross station and fanned out to various subway lines and the No. 30 double-decker bus, which left from nearby Euston station. Officials said it was still unclear whether mechanical timers or cell phones had been used to set off the blasts.

Investigators were gathering tapes from dozens of security cameras located in the stations and the subway cars. More than 104,000 people had phoned the anti-terror hotline. “We’ve got lots and lots of witnesses,” Blair said.