LONDON 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.
Police said a 29-year-old woman and three men were detained in early morning operations in West Yorkshire and in Birmingham.
The source identified the woman as Hasina Patel, whose husband Mohammad Sidique Khan was one of four young British Muslims who blew themselves up in the 7/7 attacks, killing 52 people on three London underground trains and a bus.
The four people held on Wednesday, aged between 22 and 34, are suspected of the commission, preparation, or instigation of acts of terrorism.
They were driven to London's high-security Paddington Green police station for questioning -- a short distance from the Edgware Road underground station where Khan detonated his bomb, killing himself and six others.
"They will obviously be interviewed over the course of the next few hours," a Scotland Yard spokeswoman said. Police can hold terrorism suspects for up to 28 days for questioning, but must obtain a series of warrants from a judge during that time.
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 bombers.
"This is the second phase of arrests ... but there's still plenty to do, and it's not over," a security source said.
Khan, at 30 the oldest of the bombers, had trained at an al Qaeda camp in Pakistan and is regarded as the ringleader.
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.
Police say a key focus of their investigations is to trace people who may have provided logistical support for the attacks.
"We need to know who else, apart from the bombers, knew what they were planning. Did anyone encourage them? Did anyone help them with money or accommodation?" a police statement said.
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.
Domestic spy service, MI5, last week issued a rare public defence of its operations after it emerged that its counter-terrorism agents had taken photographs and recorded conversations of Khan and another 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.