The death toll rose to 50 Friday morning as London police escalated what authorities described as the most extensive criminal investigation in British history to track down the perpetrators of Thursday's assault on the city's transportation system.

In addition to announcing the increase of 12 in the death toll, authorities said another 22 of the 700 people injured were in critical condition. Ian Blair, London's police commissioner, said he did not expect the number of dead to rise to triple digits, however.

So far, police said at a morning briefing, investigators have determined that each explosive device contained no more than 10 pounds of "high explosive" material. (The bombs on trains in Madrid in 2004, by contrast, held 17 to 22 pounds.)

While this amount could fit into a backpack or other small case, they said that they had not so far determined the nature of the container that held the explosives.

Each of the three explosive devices placed in subway cars, they said, was placed on the floor, rather than on a seat.

Police investigator Andy Hayman said forensic investigators had not yet determined the location of the explosive device on the double-decker bus that blew apart near Upper Woburn Square in the heart of the city.

But he said "there was nothing to suggest" that a suicide bomber was on the bus or that a bomb was somehow "in transit" on the bus, as opposed to the bus being a target itself along with the subway trains.

One of the explosive devices, police said, was a hundred yards into the tunnel from the Liverpool Street Station in the third carriage of a train.

The most difficult crime scene so far, Hayman said, is the subway train on which 22 people died between the King's Cross and Russell Square stations. While all living people have been removed already by rescuers, investigators have been unable to reach the train because the tunnel walls are considered unstable.

The bomb at this location was in the standing area of the first carriage.

Brian Paddick, deputy police commissioner, said the investigation would be massive, starting with collecting every piece of every explosive device at every location. Analysts would also have go through video footage from hundreds of different CCTV cameras, which proliferate in London.

"It is a very difficult case to crack," Paddick said later on CNN, far more complicated than previous investigations of Irish Republican Army terrorism. In combating the IRA, police could count on advance warnings before explosions, a relatively well-known group of suspects and a parade of informants.

Blair, police commissioner for the Metropolitan Police (popularly known as Scotland Yard), agreed that the attacks bore the "hallmarks of al Qaeda."

But he said authorities had no "specific" evidence pointing towards anyone or any group. He said they had not yet been able to establish the authenticity of various claims of responsibility issued Thursday.

Wary but determined Londoners started back to work Friday morning, using the same subway and bus system that had been the scene of terrorist bomb attacks during Thursday's morning rush hour.

Most of the system was up and running, despite closures of the stations where the attacks had taken place and several security alerts that shut down other stations during the morning.

Police acknowledged that they had lowered the level of security alert in the weeks before the attack and had dispatched 1,500 London policemen to Scotland to help with security for the Group of Eight summit there.

But they vowed a massive investigation to find the bombers.

Home Secretary Charles Clarke told the BBC investigators that other attacks could occur.

Fred Barbash reported from Washington.