British police arrested the widow of one of the London suicide bombers of July 7, 2005, along with three other suspects on Wednesday, a source familiar with the operation said.
A police statement said a 29-year-old woman and three men were detained in early morning raids in the northern region of West Yorkshire and in the West Midlands.
The source confirmed the woman was Hasina Patel, whose husband Mohammad Sidique Khan was one of four young British Muslims who blew themselves up, killing 52 people, on three London underground trains and a bus.
Khan, the oldest of the four, had trained at an Al Qaeda camp in Pakistan and is regarded as the ringleader of the group.
Revelation of his involvement in the attacks caused deep shock in his local community, where he had worked as a mentor in a primary school. He and Patel had a daughter who was a baby at the time of the attacks.
The suspects held on Wednesday, aged between 22 and 34, are suspected of the commission, preparation, or instigation of acts of terrorism.
It was the second wave of arrests within weeks in the vast and long-running investigation into the 7/7 attacks, the first suicide bombings by Islamist militants in Western Europe.
Three men were charged last month with conspiring with the four young British Muslims who carried out the bombings on three London underground trains and a bus.
This is the second phase of arrests ... but there's still plenty to do, and it's not over," a security source told media.
Police said the suspects were being taken to a central London police station to be interviewed by counter-terrorism detectives. They were searching five houses in West Yorkshire and two flats in Birmingham in central England.
Security sources say a key focus of investigations into the 2005 attacks is to trace people who may have provided logistical support to the bombers.
Peter Clarke, head of Scotland Yard's counter-terrorism command, said last month he was certain some people with knowledge of what lay behind the attacks had so far withheld information from police. He said at the time it was highly likely there would be more arrests.
Britain's domestic spy service, MI5, last week issued a rare public defense of its operations after it emerged that its counter-terrorism agents had taken photographs and recorded conversations of two of the suicide bombers, well over a year before they carried out the attacks.
The agency said the men surfaced as unidentified contacts of a group of men under surveillance in a separate plot, and there was no evidence at the time that the two were involved in terrorist activity in Britain.