Three of the four alleged London bombers traveled to this southern city last year, an immigration official said Monday, as authorities tried to determine whether extremists in Pakistan aided in the July 7 attacks.

One of the men came to Karachi (search) in July 2004, though details of when he left the country were unclear, said Shahid Hayyat, a deputy director at the Federal Investigation Agency (search). In November 2004, the other two men arrived on the same flight and took the same return flight to London in February, Hayyat told The Associated Press.

The three men were Britons of Pakistani origin, and authorities are trying to find out whether radicals in Pakistan provided them with training or other assistance in the London attacks that killed 56 people and wounded 700. The four attackers blew themselves up on subway trains and a double-decker bus, according to investigators.

The three were identified by their photos with those taken of the men as they went through immigration in Karachi.

Karachi, a transit hub for travelers to Pakistan, is the country's commercial capital and largest city, with a population of 15 million. The vast metropolis has been the scene of several terrorist attacks against foreigners, as well as minority Shiite Muslims and Christians.

Pakistani security officials have arrested several Al Qaeda operatives in Karachi, including Ramzi bin al-Shibh (search), who was caught after a shootout in September 2002.

London bombing suspect Hasib Hussain arrived at Karachi airport on July 15, 2004, on Saudi Arabian Airlines Flight SV-714. No record of his departure for London or any other destination was immediately available, and authorities were investigating his travel route, Hayyat said.

Shahzad Tanweer and Mohammad Sidique Khan flew to Karachi on Nov. 19, 2004, on Turkish Airlines Flight 1056; both left for London aboard Turkish Airlines Flight 1057 on Feb. 8, 2005.

Hayyat said he did not know what the men did during their visits, or whether they stayed in Karachi or traveled to other parts of Pakistan. "I know that our security agencies are trying to get such details," he said.

This weekend, Pakistani intelligence agents questioned students, teachers and administrators at a religious school in the eastern city of Lahore that Tanweer reportedly visited. A school spokesman denied Tanweer had been there.

In Islamabad on Monday, President Gen. Pervez Musharraf urged a gathering of youths to avoid extremist groups, saying they were not friends of Islam or Pakistan. He condemned the London attacks.

"Islam does not allow such things," Musharraf said, addressing a convention of students.

The fourth bombing suspect has been identified as Jamaican-born Briton Lindsey Germaine.