Police questioned three British men at a high-security police station Friday after their arrest on suspicion of assisting the worst-ever terrorist attack in London — the July 7, 2005, suicide bombings that killed 52 commuters.

It was the first major development in the investigation in months, but detectives insisted exhaustive work has continued quietly for the last two years.

"The investigation has never gone away," a police spokeswoman said on condition of anonymity in line with departmental policy.

Police searched homes in the Beeston suburb of the northern city of Leeds, where three of the four suspects lived. One raided property was on the same street that was home to one of the four suicide attackers — Shehzad Tanweer.

Officers also searched another property in Bromley-by-Bow in east London.

Two men were arrested Thursday as they attempted to board a flight to Islamabad, Pakistan, while the third was arrested in Leeds.

Unarmed counter-terrorist police took the men into custody, and authorities described the searches as low-key, stressing they were not looking for bombs or bomb-making equipment.

All three men were arrested on suspicion of committing, preparing or instigating acts of terrorism. "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.

A government account of the attacks released last year said it was unclear whether others in Britain had radicalized or incited the group, and that it was not known if Al Qaeda figures, or others, had assisted in planning the bombing.

Police and security officials have indicated they believed the gang would have required some level of support to carry out its attacks.

"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."

None of the men arrested Thursday have been identified. The two arrested at Manchester Airport were 23 and 30; the third arrested in Leeds was 26.

Neighbors in Leeds said the 26-year-old man was devoted to his family and two children.

The 30-year-old man was a taxi-driver for Gee Gee Cars, a firm based in Beeston, the company owner Abdul Wahweed said. He quit two weeks ago saying he was going to Pakistan to deal with family problems, Wahweed said.

Under British law, police have up to 28 days to question the three suspects — with the consent of a judge — before they must be charged or released.

No one has ever been charged over the July 7 attack, which targeted three subway cars and a bus, killing 52 people and wounding around 700. They were the first suicide bombings on European soil.

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.

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, Tanweer, 22, and Hasib Hussain, 18 — were British-born men of Pakistani descent who grew up in the ethnically mixed Leeds, about 200 miles (320 kilometers) 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.

However, a parliamentary inquiry into the attack found Khan — under a variety of aliases — was known to Britain's domestic spy agency MI5. Tanweer and Lindsay also showed up in minor records kept by security officials.

Khan and Tanweer both visited Pakistan from Nov. 19, 2004, to Feb. 8, 2005, and may have met with Al Qaeda figures there, the government report said. Khan also may have made his "martyrdom video" there, it said. Tanweer also appeared in a similar film.

"The extent of Al Qaeda involvement is unclear," the report said. Police officials have since acknowledged the terror network is likely to have played some role.

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.