British investigators were to begin questioning three suspects Friday over their alleged involvement in the worst-ever terrorist attack in London, the July 7, 2005 suicide bombings that killed 52 commuters.
Counter-terrorism police arrested the men, all British nationals, Thursday on suspicion of playing a role in the attack.
It was the first major development in the investigation in months.
Unarmed police took two of the men into custody as they prepared to board a flight for Pakistan on Thursday. The third was arrested in Leeds, the northern city where three of the four suicide attackers lived.
Police in Leeds raided five properties — including at least one on the same street that was home to one of the four suicide attackers. Authorities described the searches as low-key, and said that they were not looking for bombs or bomb-making equipment.
Officers also searched an apartment and a business in east London, though the investigation remained centered in Leeds.
"Anybody who imagined that this had simply been treated as four lone wolves, or a lone pack of wolves on July 7, 2005, is very wrong," Lord Carlile, the government's independent reviewer of terrorism laws, told the British Broadcasting Corp. "There is a lot of work going on."
The arrests came after criticism of the Metropolitan Police, whose investigation had consumed enormous resources and spanned the globe with little outward sign of progress. An official account of the attacks last year concluded the terrorist plotters who inspired and prepared the July 7 bombers were still at large.
All three men were arrested on suspicion of committing, preparing or instigating acts of terrorism. The suspects were being held at the Paddington Green high-security police station in central London. Officers from the Met's Counter Terrorism Command were to begin interrogating the men Friday.
"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.
None of the men were identified. The two arrested at Manchester Airport were 23 and 30; the third arrested in Leeds was 26.
Under British law, police will have 28 days to question the three suspects before they must be charged or released.
No one has ever been charged in the terrorist attack. Only two other men have been arrested in the case, in 2005 — one of them was released without charge and the other was charged with wasting the time of police.
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.
In a video recorded before his death, one of the suicide bombers, Mohammed Sidique Khan, 30, pledged allegiance to al Qaeda and said he was "protecting and avenging my Muslim brothers and sisters."
Three of the bombers — Khan, Shehzad Tanweer, 22, and Hasib Hussain, 18 — were British-born men of Pakistani descent who grew up in the ethnically mixed Leeds, 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 caused soul-searching across Britain and raised fears of a threat from homegrown terrorists.
A government investigation of the attack found that Khan and Tanweer had both visited Pakistan from Nov. 19, 2004, to Feb. 8, 2005, and may have met with al Qaeda figures there. It said Khan also may have made his "martyrdom video" there.
Since then, 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.
Six men are currently on trial for allegedly attempting to bomb three London subway trains and a bus two weeks later — on July 21, 2005. The bombs failed to explode, and the men have been accused of trying to carry out a copycat of the July 7 attack.