NAIROBI, Kenya — The failed bombing of a nightclub Monday in the Ugandan capital of Kampala, which came just one day after twin explosions killed more than 70 people there, highlights the growing sophistication of African Islamist groups and raises concerns about their ability to strike abroad, security officials and analysts said.

The Ugandan police, who have been joined by a team of FBI investigators, said evidence found at the scene of Monday's failed bombing indicated the attack was most likely a planned suicide bombing similar to the ones the day before, which targeted Ugandans watching the World Cup across the city.

Somalia’s Islamist insurgents have claimed responsibility for the bombings, marking the first time that Al Shabaab, an extremist group linked to Al Qaeda, has launched attacks outside their own country’s borders.

Although a frequently used tactic in conflicts in Afghansitan, Chechnya and Iraq, suicide attacks were rare in Africa until Al Shabaab began using them in 2006. Since then, almost two dozen suicide bombings have been carried out in Somalia, said Stig Jarle Hansen, associate professor of International Relations at Norway’s University of Life Sciences.

Hansen, who traced the origins of Somali suicide bombings in a forthcoming report, warned that such attacks are becoming more frequent and more deadly.

“Suicide bombings in Somalia have been underestimated,” he said in an interview. “It is now the longest, most sustained and deadly suicide campaign ever to have taken place south of the Sahara.

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Sunday’s attacks in Uganda are believed to be in retaliation for the deployment of thousands of Ugandan troops by the African Union peacekeeping mission in Mogadisu, Somalia's capital. But the bombings also underscore Al Shabaab’s growing international reach and its increasing use of Al Qaeda tactics.

In February the Somali insurgents declared their allegiance to Al Qaeda but they have long shared a similiar extremist ideology.

Al Shabaab’s top leaders are Somalis who trained and fought in Afghanistan and Pakistan, analysts said. The group has met with Al Qaeda operatives and learned from them — most famously with Fazul Abdullah Mohamed, Al Qaeda’s East Africa leader who is wanted by the FBI for the 1998 bombings of U.S. embassies in Kenya and Tanzania and the subsequent bombing of an Israeli-owned hotel in Mombassa, Kenya in 2002.

“Fazul is the most veteran of the veterans of East Africa’s Al Qaeda wing,” said Rashid Abdi, an analyst at the International Crisis Group in Nairobi and an expert on the structure of Islamist fighting groups in Somalia.

A recent Crisis Group report described Fazul as “commander in chief of Al Shabaab” and part of “a foreign jihadi cabal” running the insurgency alongside three top Somali-born militants — all of whom fought and trained in Afghanistan in the 1980s and 1990s.

Hansen said Fazul is acting as an instructor, leading training camps for suicide bombers and bombmakers — which analysts said explains the growing sophistication of Al Shabaab's attacks.

Western intelligence officials are also increasingly concerned about the growing number of radicalized young men arriving in Somalia from as far as the U.S. and Europe.

Security forces foiled an Al Shabaab bomb plot last year in Australia and in January an axe-wielding Somali man attempted to murder a Danish cartoonist who had published drawings of Prophet Mohammed.

Of particular concern to the FBI investigarors is the more than 20 young Somali-Americans who have disappeared from their homes in Minneapolis, Minn. in recent years. FBI investigators believe the youngsters were radicalized in the U.S. before traveling to Somalia to join Al Shabaab.

In October 2008, in the northern Somali town of Bossaso, Shirwa Ahmed, 26, became the first U.S. citizen to carry out a suicide bombing on behalf of Al Shabaab.

Another Somali-American is believed to have taken part in a suicide attack that killed dozens of peacekeepers in Mogadishu in the fall of 2009 while a Danish-Somali man was responsible for the suicide bombing at a university graduation ceremony in December.

“It raises the question of whether these young men will one day come home, and, if so, what they might undertake here,” FBI Director Robert Mueller, speaking about the missing American men last year, said.

Sunday’s bombings, and Monday's failed attempt, have only increased fears that Al Shabaab members could soon be sophisticated enough to threaten the U.S., which backs Somalia’s shaky government and is home to large Somali communities.

“East Africa-based Al Qaeda leaders or Al Shabaab may elect to redirect to the [U.S.] some of the Westerners … now training and fighting in Somalia,” Dennis Blair, President Barack Obama's former intelligence chief, said earlier this year.

Hansen agreed.

“Al Shabaab is not technologically advanced but ideologically their training is getting more and more rigid. So at some point you might just get people coming [to Somalia] then returning to the West with a changed mindset to do things in their home country," he said. "This is absolutely a real danger."

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