British officials are asking the public for help in tracking down the people suspected of setting off four small explosions Thursday in the city's transit system.
Authorities released images of the suspects during a news conference Friday in London. Three of the images showed the suspects at train stations, and the other showed a suspect on a bus.
The city's police commissioner, Ian Blair, also said the fatal shooting by police of a man at a London subway station earlier Friday was directly linked to the ongoing terrorist investigation in the city.
Police say they are also searching some houses in London in what they describe as a fast moving investigation.
Meanwhile London commuters began returning to subway trains and buses while police continued their search for the suspects in Thursday's lunch hour attacks, which caused no serious injuries.
Authorities say they have considerable evidence in their investigation of the explosions on three subway trains and a bus that came two weeks after similar bombings on the transit system killed 56 people.