Updated

At least three of the four suspects in the July 21 attempted bombings on the London subway and a bus were born in East Africa, where Al Qaeda-linked groups still operate and may be growing in strength, according to a new assessment by counterterrorism experts.

The attackers, at least two of them naturalized British citizens, were born in Somalia, Ethiopia and Eritrea and there is no evidence they have been back there recently. But East Africa has several indigenous terror groups and has suffered three Al Qaeda (search ) attacks since 1998.

"There is a genuine threat, there is no doubt the networks are still present and they retain the capacity to strike again," said Matt Bryden, an East Africa (search) analyst for the think-tank International Crisis Group (search ). "On the other hand, much more is known about these groups, there has been an intelligence surge in the last few years, they are kept under pressure."

Usama bin Laden (search ) moved to East Africa in 1991 at the invitation of Hassan al-Turabi, an Islamic fundamentalist once considered the spiritual, if not de facto, leader of Sudan. Bin Laden brought with him Afghan war veterans, millions of dollars and plans to start Al Qaeda.

In 1992, bin Laden dispatched some of his deputies to Somalia, where a U.S.-led peacekeeping operation was under way. The operatives trained members of a Somali Islamic group called al-Itihaad Islamia, according to former Somali fighters.

Al-Itihaad members took at least partial credit for shooting down a Black Hawk helicopter in 1993, a battle that left 18 U.S. soldiers and hundreds of Somalis dead. Bin Laden considered the subsequent withdrawal of U.S. troops from Somalia his first victory against America.

During this time, Al Qaeda operatives reached out to other Islamic fundamentalist groups in the region, including Eritrean Islamic Jihad and the Ethiopian branch of al-Itihaad.

Under U.S. pressure, the Sudanese government expelled bin Laden in 1996, forcing him to move to Afghanistan. But Al Qaeda left cells behind in East Africa, and two of them attacked the U.S. Embassies in Nairobi, Kenya and Dar es Salaam, Tanzania in 1998, killing 12 Americans and more than 200 Africans. Sunday was the seven-year anniversary of the Aug. 7 attacks.

The Al Qaeda controller for those attacks, Fazul Abdullah Mohammed, escaped Kenya to Somalia following the bombings, and in 2002 organized the car bombing of an Israeli hotel in Mombasa, after which he again went to Somalia, suspects have told interrogators.

While al-Itihaad was largely destroyed or disbanded by Ethiopian troops fighting inside Somalia by 1997, some of its members have regrouped under new guises and have begun to grow in strength, according to an International Crisis Group report released in July.

Somalia, divided into warring fiefdoms, remains fertile ground for terrorists.

The United Nations and ICG have identified Sheikh Hassan Dahir Aweys, a former al-Itihaad member and now leader of Somalia's Islamic courts, as a key figure. He is also on a U.S. list of suspected Al Qaeda members.

In his ICG report on Somalia, Bryden identified an Aweys associate who trained in Afghanistan, Aden Hashi Ayro, as the leader of a new "small but ruthless network based in Mogadishu." Ayro's group has been implicated in a number of assassinations in Somalia, the July report said.

The Islamic courts have heavily armed militias and financial support from powerful Somali businessmen, who try to keep their political activities secret, Mogadishu residents have told The Associated Press.

Aweys has refused to participate in forming a new government for Somalia and has threatened a religious war if foreign troops are brought in to help disarm the rival militias.

A counterterrorism force deployed in Djibouti by the U.S. has concentrated on improving the ability of East African governments to fight terrorism. The Combined Joint Task Force-Horn of Africa provides intelligence-gathering help, regional cooperation and border protection, as well as humanitarian projects to improve the U.S. military's image among Muslims.

Bryden said that while there is still a clear threat of more terrorist attacks in East Africa, important strides have been made.

"There has been an investment in border controls, the computerization of immigration information and the upgrading of security forces," Bryden said. "The sharing of intelligence among countries in the region has been stepped up considerably."

Still, no one has been convicted in any East African country of a terrorist act, and most East Africans deny there is a terrorist threat in the region.

The men under investigation in London are the first East Africans suspected of involvement in a terrorist attack outside the continent since the Sept. 11 attacks.