July 22, 2005 — -- Clues found at the scenes of Thursday's attacks are leading British police closer to the suspected bombers.
Law enforcement officials tell ABC News clues found in and around the unexploded bombs are pointing them toward four men, whose photographs, taken by video surveillance cameras, were released today.
Late Thursday, a set of fingerprints was found on the upper deck of the No. 26 bus. The prints, lifted from the unexploded backpack bomb, matched others on file.
"Clearly if his fingerprints were identified as a result of a database check," explained Jack Cloonan, a former FBI agent and now ABC News consultant, "he's been known to law enforcement at some level at some point."
The identity of the bus bomber led police to monitor three buildings in London, including one near the Stockwell subway station.
Officials tell ABC News the suspect shot dead today left that building this morning and was being followed by a special police squad.
"This shooting is directly linked to the ongoing and expanding anti-terror operation," Ian Blair, commissioner of the Metropolitan Police, said of the incident.
Other clues are being found within the unexploded bomb itself. The yellow material, or the explosive powder which failed to detonate, is a chemical match to the explosive used two week ago. Officials believe it failed to explode yesterday because it had degraded over time.
Officials also tell ABC News the detonation switch was as simple as a Duracell 9-volt battery wired to a blasting cap and a light switch, which is the classic combination for a suicide bomb.
"These are very dangerous men. They have decided to become 'shaheed' or martyrs. The British are in a race against time," said Cloonan. "They need to find out where the rest of this crew is and whether they have any more devices because they can get on the subway any time and commit suicide."
As to whether the men may strike again, Cloonan replied, "Absolutely. Unquestionably."
ABC News' Chris Isham and David Scott contributed to this report.