Opinion: Al-Shabaab's American allies - CNN.com 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 (CNN) -- 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. Opinion: How Al-Shabaab picks its targets 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. 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). Opinion: What threat do foreign jihadists pose? 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.