Story highlights Peter Bergen: Of al Qaeda's affiliates, Al-Shabaab has had deepest links to the U.S.

He says 15 Americans have died fighting for Al-Shabaab, including suicide bombers

Bergen: Ethiopian invasion of Somalia has played a role in stirring recruitment

He says FBI, Justice Department have cracked down on American support

Of all al Qaeda's affiliated groups, the Somali terrorist organization Al-Shabaab has over the past several years had the deepest links to the United States. Some 15 Americans have died fighting for Al-Shabaab, as many as four of them as suicide bombers in Somalia, and an American citizen even took up a leadership role in the group.

Al-Shabaab has also found supporters in places as diverse as Seattle, St. Louis, San Diego, Minnesota, Maryland, Ohio and Alabama.

Al-Shabaab had particular success recruiting Somali-Americans to its cause after the Ethiopian army invaded Somalia in 2006, which Al-Shabaab cast as Somalia being taken over by a "crusader" army. Ethiopia is a majority Christian nation.

The largest group of American citizens and residents who have provided manpower and money to Al-Shabaab reside in Minnesota. According to a count by the New America Foundation, 22 residents of Minnesota have funded or fought with Al-Shabaab during the past four years.

Peter Bergen

Three of them provided funds to Al-Shabaab, and 19 have been indicted for traveling to fight in Somalia or have died in the war there.

The story of Minnesotan support for Al-Shabaab began in late 2007, when Cabdulaahi Ahmed Faarax, an American citizen of Somali descent in his early 30s, and several other men met at a Minnesota mosque and discussed traveling to Somalis to fight for Al-Shabaab.

Faarax told the group that he had "experienced true brotherhood" while fighting in Somalia and that "jihad would be fun" and they would " get to shoot guns ," according to the U.S. Justice Department.

That meeting resulted in seven men traveling from Minnesota to Somalia to fight for Al-Shabaab in late 2007.

One was Shirwa Ahmed , a 26-year-old naturalized American citizen. Ahmed became the first American to conduct a suicide attack when he drove a truck loaded with explosives toward a government compound in Puntland, northern Somalia, blowing himself up and killing 20 other people in October 29, 2008. He is buried in a cemetery in Burnsville, a suburb of Minneapolis.

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Other American suicide attackers would follow. In early June 2011, Farah Mohamed Beledi, 27, of Minneapolis detonated a bomb, becoming one of two suicide attackers responsible for killing two African Union soldiers in Somalia, according to the FBI.

The third American to conduct a suicide attack was Abdisalan Hussein Ali, a 22-year-old from Minneapolis who took part in a strike on African Union troops in the Somali capital of Mogadishu on October 29, 2011.

There may even have been a fourth American suicide attacker in Somalia. On September 17, 2009, two stolen U.N. vehicles loaded with bombs blew up at the Mogadishu airport, killing more than a dozen peacekeepers of the African Union. The FBI suspects that 18-year-old Omar Mohamud of Seattle was one of the bombers.

For those Americans who have traveled to Somalia to fight for Al-Shabaab, it has often proved to be a one-way ticket. A 2011 report by the U.S. House of Representatives Committee on Homeland Security found that at least 15 Americans had died while fighting for Al-Shabaab (as well as three Canadians).

Some of the young men who volunteered to fight in Somalia had grown up in the Cedar-Riverside neighborhood of Minneapolis, which is one of the poorest places in the United States. In recent years, Somali-American family incomes there averaged less than $15,000 a year, and the unemployment rate was 17%.

Al-Shabaab's American support network also extended well beyond Minnesota.

Basaaly Saeed Moalin, a cabdriver in San Diego who was in contact with an Al-Shabaab leader, was convicted of sending funds to the group along with three co-conspirators this year.

In St. Louis, Mohamud Abdi Yusuf pleaded guilty in 2012 of providing funds to Al-Shabaab.

A resident of Ohio, Ahmed Hussain Mahamud, was indicted in 2011 for funding Somali-Americans traveling to join Al-Shabaab.

Al-Shabaab's support network in the United States has reached beyond the Somali-American community. Ruben Shumpert , an African-American convert to Islam from Seattle, was killed in Somalia in 2008.

A former U.S. soldier, Craig B. Baxam, 24, of Laurel, Maryland, was arrested by Kenyan authorities in December 2011 as he tried to make his way to Somalia to join Al-Shabaab, which he told FBI agents he considered to be a religious duty.

Omar Hammami of Daphne, Alabama, grew up Baptist and converted to Islam when he was in his teens. In a lengthy autobiography that Hammami posted online last year entitled "The Story of an American Jihadi," he explained his long journey from growing up Christian in a small town in Alabama to fighting on the front lines in Somalia with Al-Shabaab.

The journey began with a life-changing trip to Syria, the homeland of his father, when he was 15 that sparked his interest in Islam. Hammami wrote in his autobiography (PDF), "when I came back from that vacation, I had become a different person."

Over the past several years, Hammami rose up the ranks in Al-Shabaab, becoming an important leader. Disputes with other Al-Shabaab leaders led him to split off from the group. He was killed this month, probably by members of Al-Shabaab, according to Islamist websites.

In the wake of many of these developments, for the past three years, the Justice Department and the FBI have engaged in a serious effort to crack down on U.S. support for Al-Shabaab, in particular in Minnesota, in an effort codenamed Operation Rhino.

This seems to have had some success, as the number of Americans indicted for supporting Al-Shabaab on the front lines or with their wallets has dropped sharply since the launch of Operation Rhino.

Over the weekend, Al-Shabaab issued a list of nine names it claimed were among the attackers who carried out its deadly assault on the upscale Westgate Mall in Nairobi, Kenya. Al-Shabaab alleged that three of the attackers were from the United States. The FBI is looking into whether these claims are true.

Whether or not any Americans played a role in the massacre in Nairobi that has claimed 62 lives, there is a deadly history of American support for Al-Shabaab.