Police also said they had uncovered no evidence suicide attackers had set off the explosions, but stressed they were still in the early stages of what promises to be an arduous investigation.
Law enforcement officials declined to respond to questions about a U.S. official's statement that evidence indicating timers were used was found in the debris. London police also played down the possibility the devices were detonated by remote control using cell phones, instead asking the public for patience Friday as their investigation picks up momentum.
CBS News Correspondent Jim Stewart reports
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.
Whoever placed the bombs put them on the floor in three Underground cars, and either on the floor or on a seat of one of London's red, double-decker buses, the city's police commissioner said at a news conference.
"We have absolutely nothing to suggest this was a suicide bombing attack although nothing at this stage to rule that out," Sir Ian Blair, commissioner of the Metropolitan Police, said.
Police said they had found no bombs other than the four that exploded. Police destroyed two suspicious packages in other areas in controlled explosions, but Blair said they turned out to be harmless.
Media reports of additional explosives could be attributed to the initial confusion about the number of bombs, said Assistant Commissioner Andy Hayman.
"Initially, the forensic investigation suggests that each device used had less than 10 pounds of high explosives," Hayman said. The weight of explosives was smaller than recent bombs detonated in the Middle East.
Hayman appealed for patience as the investigation proceeds.
"Our people are working under the most extreme circumstances," he said — including amid fears that the tunnel will collapse on top of them at the blast site near the Russell Square tube station, where bodies lay uncollected a day later as engineers studied the area.
Blair said no arrests have been made but officials have "lots and lots" of leads.
How the bombs were detonated remained an open question Friday.