Police made a major break in the case in just five days with the help of footage from surveillance cameras and the discovery of IDs at the bomb sites. But it was a family's heartache that apparently helped point them in the right direction.
The mother of one of four suspects - identified in press reports as 19-year-old Hasib Hussain - called a police hotline to report him missing about 12 hours after the Thursday bombings. He'd told his parents he was planning a day out in London with his friends.
Hussain is believed to have died on board the No. 30 bus, which exploded nearly an hour after three bombs devastated London's Underground during the morning rush hour on Thursday. A witness who got off the crowded bus just before it exploded told The Associated Press that he saw an agitated man in his 20s fiddling anxiously with something in his bag.
The Times newspaper in London said Mrs. Hussain gave her son's name and described what he was wearing the last time she saw him. The burned and bloodied clothes of one young man killed on the bus matched the description and investigators noted that the wounds the teenager sustained meant he had to have been close to the bomb, according to the newspaper.
Teams of police and computer specialists were then reportedly told to look for a distinctive-colored top worn by Hussain in the countless hours of surveillance tapes being examined and to concentrate on film taken from the King's Cross station. That helped narrow the search.
Video reportedly showing Hussain and three other suspects toting backpacks and parting ways in King's Cross station just before the explosions provided the critical clue, along with the suspects' credit cards and IDs, found in the wreckage of three of the four bomb blasts.
Peter Clarke, head of the Metropolitan Police anti-terrorist branch, said one of the suspects had been reported missing by his family shortly after 10 p.m. Thursday, but he didn't elaborate on the phone call.
"As regards the man who was reported missing, some of his property was found on the Route 30 bus in Tavistock Square," Clarke said at a news conference.
Detectives on Saturday revised their initial timeline of events, ascertaining from a review of the subways' computer and electrical systems that the bombings on three Underground trains occurred within a minute of one another, at 8:50 a.m., not over a 26-minute span as they had initially thought.
That also allowed them to target the most relevant sections of the hundreds of hours of closed-circuit television footage they'd seized. Cameras monitor much of central London, including 1,800 in train stations and 6,000 in the Underground network.
At around 8 p.m. Monday, investigators found the images they were looking for.
Four young men carrying backpacks chatted casually in King's Cross station at about 8:30 a.m. - 20 minutes before the subway explosions - then separated. At rush hour in the busy station, a major hub for north London, they would have attracted little notice.
Authorities initially thought Hussain might have been a victim and they sent family liaison officers to meet his parents. They would have obtained photos and other identifying details to help track down the vanished teen - pictures that matched the CCTV images.