LONDON - Counter-terrorist police arrested three men Thursday in the 2005 suicide attacks on the London transit system, the first major development in the investigation in months. Two of the suspects were detained as they prepared to board a flight to Pakistan.

Police stand outside a house in Colwyn Road, Beeston, England, on Thursday March 22, 2007, following the arrest of people on suspicion of committing, preparing or instigating acts of terrorism. [AP]

The third man was arrested in Leeds - the northern city that was home to three of the four bombers. Police also raided five properties in the city - at least one on the same street where one of the bombers lived.

"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 terror laws, told the British Broadcasting Corp. "There is a lot of work going on."

No one has ever been charged in connection with the bombings, which were the deadliest attack on London since World War II. The four bombers and 52 commuters died in blasts on three subway trains and a double-decker bus, and more than 700 people were injured.

Thursday's 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 plotters who inspired and prepared the bombers were still at large.

All three men were arrested on suspicion of committing, preparing or instigating acts of terrorism. Police said the suspects were being taken to a high-security central London police station for questioning.

"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 aged 23 and 30 and the third was 26. Under British law, police will have 28 days to question them before they must be charged or released.

Authorities described the property searches as low-key and said 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.

Only two other men have been arrested in connection with the case in 2005. One was released without charge and one 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 coordinated attacks on London commuters were the first suicide bombings on European soil. The attack was followed two weeks later by a copycat plot in which four bombs failed to detonate.

In a video recorded before his death, one of the suicide bombers, Mohammed Sidique Khan, 30, pledged allegiance to al-Qaida 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 November 2004 to February 2005, and may have met with al-Qaida 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 men have been accused of attempting a copycat of the July 7 attack.