Police rounded up seven Islamic militants in Pakistan to determine whether the London bombing plot stretched to South Asia, and Prime Minister Tony Blair asked British Muslim leaders Tuesday to weed out extremists blamed for radicalizing their young followers.

Investigators are trying to find out whether any militant group or individual provided three of the four London suicide bombers who visited Pakistan last year with training or other assistance in the July 7 bombings aboard three subways and a bus that killed at least 56 people and injured 700.

"We are holding a few militants who are suspected of having links to the London suicide bombers," said Tariq Saleem, police chief in the town of Lahore (search). Officials want to determine whether the "London bombings have any tentacles in Pakistan, especially in Lahore," he said.

London Times Bombing Coverage

Saleem did not name the suspects or say how many were detained.

Other police officials said on condition of anonymity that the seven detained men were from two outlawed militant groups, Lashkar-e-Jhangvi (search) and Jaish-e-Mohammed (search). Both are Al Qaeda-linked, and some of their supporters have been arrested for trying to assassinate President Gen. Pervez Musharraf (search).

Five of the detained men were picked up in eastern Punjab province in recent days, and two were caught overnight in southern Sindh province, the officials said.

On Monday, Shahid Hayyat, deputy director at Pakistan's Federal Investigation Agency, told The Associated Press that three of the London suspects traveled to the southern port of Karachi last year — Hasib Hussain (search), 18, in July 2004, and Mohammad Sidique Khan (search), 30, and Shahzad Tanweer (search), 22, in November. All three were Britons of Pakistani origin. Hayyat said the purpose of their trip was still unclear but authorities were investigating it.

In Britain, pictures of the three men taken by immigration officials at the airport in Karachi were widely printed and broadcast Tuesday.

Pakistani intelligence officials have said Tanweer, born in Britain to Pakistani parents, stayed briefly at a religious school in Lahore. They said Tanweer met Usama Nazir, a Pakistani arrested in November 2004 for helping plan a 2002 grenade attack on an Islamabad church that killed five people, including two Americans. Nazir, a member of the outlawed Jaish-e-Mohammed, told officials he met Tanweer last year in Faisalabad, southwest of Lahore.

Many militant groups maintain clandestine offices in Lahore, near the border with India in eastern Punjab province, and some Al Qaeda operatives have been arrested there.

In Britain, the Times of London reported Pakistani authorities know the identity of a British-born man whom London investigators believe may have masterminded the bomb plot.

Al Qaeda organizers around Europe may also have provided organizational help.

Security forces in London remained vigilant. British Transport Police dispatched dogs to search for explosives on the Underground. Dogs have been used before on the train that connects Heathrow airport to the capital, but police said this was the first time they were being deployed on the subway.

Meanwhile, Blair met two dozen representatives of the Muslim community to discuss anti-terrorism legislation his government plans to introduce this year. The leaders fear the laws target their community.

"It's fair the government should ask itself whether policies such as those involving the Iraq war have contributed to this," said Dr. Zaki Badawi, head of the Muslim College. "We need a partnership between government and Muslims to show people they are not being ignored and that their concerns will be heard."

Opposition Conservative Party leader Michael Howard (search), who attended the meeting, said the strongest message was "the responsibility of the Muslim community for reaching out to those who have been the targets of the merchants of evil and hatred."

The attacks have also raised troubling questions over possible British intelligence failures.

The New York Times reported Tuesday that a panel of British intelligence and law enforcement officials had played down the possibility of an attack less than a month before it occurred.

The newspaper said it had obtained a confidential threat report by the Joint Terrorist Analysis Center (search) which prompted the government to lower its assessment of the likelihood of an attack one level, from "severe defined" to "substantial."

But the report suggested Iraq was acting as a "motivation and a focus of a range of terrorist related activity."

Blair's government has sharply disputed the contention that Britain's alliance with the United States in the Iraq war may have motivated the bombers.

Elsewhere, British officials on Tuesday investigated whether one of the London suicide bombers used perfume bottles to make his bomb even deadlier.

Detectives are trying to retrace the four bombing suspects' footsteps, including a report in the Daily Mirror that Jamaican-born Jermaine Lindsay (search) — one of the suspects — reportedly bought hundreds of dollars worth of perfume days before the attacks.

Scotland Yard refused to comment.

The metal perfume bottles could have been transformed into shrapnel in the blast. Investigators are still trying to determine what material was used in the four bombs detonated aboard three subways and a bus in the capital. At least 56 people were killed.

At least one person in Britain has been arrested in connection with the bombings. The man, whose name has not been released, was detained last week in a series of raids on homes in West Yorkshire, northern England, where three of the suspected bombers lived.

News reports have said a Briton of Pakistani origin suspected of links to Al Qaeda and involvement in the bombings had entered Britain two to three weeks before the attacks and flown out the day before.

"This would indeed be evidence of an enormous failure" of intelligence, if true, said Charles Shoebridge, a security analyst and former counterterrorism intelligence officer.

The Home Office, which speaks for the domestic intelligence service MI5 (search), declined to comment on the suggestion that agents had missed the suspect, whose identity is not known, or on reports that at least one of the suspected suicide bombers was investigated last year by MI5.

Questions are intensifying over the revelation that MI5 agents reportedly determined that Khan was not a threat to national security and decided against putting him under surveillance after checking him out in connection with an alleged plot to blow up a truck bomb in London.

British intelligence reportedly found that Khan, 30, a teaching assistant at a northeastern England primary school, had visited the home of a man linked to an alleged plot to blow up a London target, possibly a Soho nightclub, with a fertilizer bomb.

In that investigation, known as Operation Crevice (search), detectives arrested eight suspects across southern England and seized half a ton of ammonium nitrate, a chemical fertilizer used in many bomb attacks.

Intelligence bosses face a tricky task in choosing how to allocate their limited resources for tasks like surveillance, Shoebridge said.

Nonetheless, he said, "had the assessment of the available intelligence regarding Khan been different, so might also have been the outcome of July the 7th," when the attackers blew up three London subways and a double-decker bus, killing 56.

John Carnt, a former Scotland Yard detective superintendent with expertise in counterterrorism and covert surveillance, said intelligence agencies are so bombarded with information it can be hard to home in on any one individual.

Khan's "might have been one name amidst many other names, and there may have been nothing else that added weight to it," said Carnt, now managing director of Vance International Ltd., a London-based security and intelligence company. "You've got bits of information coming across your desk. It can be difficult to identify which bit to pay closer attention to."