British counter-terrorist police arrested three suspects on Thursday over the July 7, 2005, London transit bombings, which killed 52 commuters and four bombers.
No one has ever been charged in connection with the bombings, the deadliest attack on London since World War II.
The Metropolitan Police said two men, aged 23 and 30, were arrested at Manchester Airport in northwest England as they prepared to board a flight to Pakistan. The third man, 26, was arrested at a house in Leeds, northern England, where police were searching five homes.
All were arrested on suspicion of committing, preparing or instigating acts of terrorism. The force said the suspects were being taken to a central London police station for questioning.
The houses being searched were all in Beeston, a working-class area of Leeds that was home to three of the July 7 bombers.
Officers also were searching an apartment and a business in east London.
Two people were arrested in Britain in 2005 in connection with the July 7 bombings. One was released without charge, and the other was charged with wasting police time.
Magdy el-Nashar, 33, an Egyptian chemist who had lived in Leeds, was detained in Cairo after the bombings and freed weeks later after Egyptian authorities said he was not linked to the attack.
The July 7 attacks on three subway trains and a double-decker bus killed 52 people and wounded more than 700. They were the first suicide bombings on European soil.
Three of the bombers — Mohammed Sidique Khan, 30, Shehzad Tanweer, 22, and Hasib Hussain, 18 — were British-born men of Pakistani descent who grew up in the ethnically mixed Leeds area, about 200 miles north of London. The fourth, Germaine Lindsay, 19, was born in Jamaica and raised in Britain.
The fact that seemingly unremarkable British youths could become suicide bombers cause soul-searching across Britain, and raised fears of a threat from homegrown terrorists.
The police investigation seemed to have stalled, and an official account of the attacks published last year concluded that the plotters who inspired and prepared the bombers were likely still at large.
The Metropolitan Police said they remained "determined to follow the evidence wherever it takes us to identify any other person who may have been involved, in any way, in the terrorist attacks."