As the gunmen from Al-Shabab, armed with automatic weapons and rocket-propelled grenades, slaughtered shoppers and families in Nairobi’s Westgate mall, the group live-tweeted its satisfaction with the hostage taking and carnage: “Like it or loathe it! our mujahideen confirmed all executions were point blank range!”

The macabre social-media commentary continued via various Twitter accounts throughout the weekend, and as Kenyan soldiers assisted by Israeli special forces began to try to clear the huge complex and rescue dozens of hostages, an English-language Twitter account purporting to speak for Al-Shabab boasted of more attacks to come. It also claimed to identify 10 of the gunmen, including three Americans, all aged in their 20s, as having taken part in the operation.

Another Arabic-language Twitter account that journalists have used to communicate with the Somali al Qaeda affiliate in the past confirmed the names of the Americans. All have Somali-sounding names, and two of them are alleged to be from the Minneapolis–St. Paul area in Minnesota, home to a large Somali-American community, from which Al-Shabab has been actively seeking to recruit in the past.

A U.S. counterterrorism source told The Daily Beast that the FBI is investigating the alleged American participants. While it won’t be known until the siege is over whether Americans were among the gunmen, there is an “increasing fear they were,” the source said. The two-day assault and siege on the upscale Westgate mall is the single biggest attack in Kenya since an al Qaeda cell bombed the U.S. Embassy in Nairobi in 1998, killing more than 200 people.

The Minneapolis–St. Paul area has one of the world’s largest Somali populations outside Mogadishu, with more than 80,000 residents believed to have originated from Somalia. Since 2007, community leaders have warned of active Al-Shabab recruitment at mosques in the Twin Cities and said two dozen young Somali-Americans have been lured to jihadist training camps overseas.

This year, Al-Shabab posted a 40-minute recruitment video, “Minnesota’s Martyrs: The Path to Paradise,” that follows three Americans from the Twin Cities as they go to training camps in Somalia and die for the jihadist cause. One of the young men urges others to follow, saying, “This is the real Disneyland. You need to come here and join us!” The narrator praises the trio as the “Minnesotan martyrs” whose “decisive moment” came when they were martyred in a jihad against foreign troops in Somalia.

One of the three featured was Troy Kastigar, a convert to Islam who was killed in Mogadishu in September 2009, about 10 months after arriving from Minnesota, according to the video. Three other Somali-Americans—Abdisalan Hussein Ali, Farah Mohamed Beledi, and Shirwa Ahmed—became Al-Shabab suicide bombers, dying in blasts in Mogadishu in recent years.

In testimony before the House Homeland Security panel in March 2011, Abdirizak Bihi, a Minneapolis community leader and uncle of a young Somali-American who was recruited by Al-Shabab and died in fighting in Somalia, warned of the extent of radicalization of young Somali-Americans in the Twin Cities.

“Looking back, my sister and I realized [along with the other mothers] that these young men had been behaving very strangely within the last three or four months before they went missing, spending most of their time at the mosque, even sleeping overnight and during the weekends there,” Bihi told the committee. “They appeared pensive and spent hours alone thinking to themselves, and wouldn’t leave the mosque. We would never have guessed that our kids had been brainwashed already and recruited to fight for Al-Shabab in a jihadist war that was killing other innocent Muslim Somalis thousands of miles away.”

According to Bihi, families of the young men who were recruited asked mosque leaders for help but were rebuffed.

Al-Shabab, in addition to the three Americans, named a Canadian, a Swede, two Britons, and a Finn as among the gunmen in the deadly mall assault. It also confirmed reports that a female jihadist took part in the attack, the first mounted by Al-Shabab outside Somalia’s borders. That confirmation has prompted speculation in the British media that the woman might be Samantha Lewthwaite, the widow of one of the London 7/7 attackers who joined al Qaeda.

Al-Shabab was forced to change Twitter accounts over the weekend as the social-networking service sought to shut down the jihadist group’s handles following complaints from other social-media users. On an Arabic-language account, Al-Shabab leaders said they were satisfied with the outcome of “this operation.”