The Islamist extremists from Somalia's al Qaeda affiliate, al Shabab, staged a deadly attack on the upscale Westgate Mall in Nairobi over the weekend, and unconfirmed reports have said that three of the 10 to 15 terrorists were Americans.

Al Shabab has succeeded in recruiting dozens of people from Somali expatriate communities in the U.S. and the U.K., mostly disenchanted young men who had a hard time adapting to life in the West. The group now claims to have 50 American members, and House Homeland Security Committee Chairman Michael McCaul (R-Texas) says that has raised "grave concern" that al Shabab might some day attempt an attack on American soil.

"When you have 50 Americans, up to 50 Americans, trained to fight the war in terrorism, the idea that they could come back to the United States is a real valid concern," McCaul says.

Intelligence experts say the attack at Kenya's Westgate Mall suggests that al Shabab, increasingly weakened at home by an offensive led by African Union troops, including Kenyans, is starting to lash out abroad to show it is still a force to be reckoned with.

Up to now, terrorism experts say, American recruits have played mostly a symbolic role in al Shabab. Its most high-profile outsider — an Alabama man named Omar Hammami — was reportedly killed last week by al Shabab after he made a break with the group. Matt Berman at National Journal notes that experts have detected chatter among al Shabab leaders about how they have squandered and mistreated their Western assets — something that might suggest they are looking to find ways to use the foreigners to inflict greater damage.

And the recruits — with their Western citizenship, connections, and on-the-ground knowledge — pose an outsized potential threat to the U.K. and the U.S... [With] relatively easy access through U.S. or U.K. borders, it's not crazy to imagine that al Shabab could be capable of launching an attack on a soft target by a gunman abroad — like the attack raging in Kenya. [National Journal]

The scope of such an attack, however, would be limited. While al Shabab is allied with al Qaeda, it does not have the latter organization's reach or scope. Al Shabab lacks a sophisticated international network, and its focus is pushing for fundamentalist Islamic rule on its home turf. That, says John Campbell at the Council on Foreign Relations, "makes a future 'twin towers' style attack on the United States highly unlikely. However, that does not preclude a small band of the home-grown disaffected actors from wrecking mayhem and appealing to an international context, as has already happened in the United Kingdom."

The group has found fighters or financial backers from Seattle to Maryland, with particular success in Minnesota (Minneapolis–St. Paul has an estimated 80,000 Somali immigrants). Al Shabab this year even posted a 40-minute recruitment video, Minnesota's Martyrs: The Path to Paradise, that follows three Americans as they leave the Twin Cities, go to terrorist training camps in Somalia, and die in battle.

At least four American recruits have conducted suicide attacks against African Union peacekeepers, notes Peter Bergen at CNN. "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 Shabab."

But how big is the threat, really? James Fergusson at The New York Times notes the number of American recruits has dropped over the past five years, and that the vast majority of Somalis who have moved to America are trying hard to make it here, and embrace their new home. He says:

I remain, however, unafraid of Somalis, least of all of Americanized ones. I spent time in Minnesota in 2011 — the Twin Cities is home to the greatest concentration in the United States of Somalis in exile — and uncovered this reassuring truth: Hotheads inclined to support the Shabab may exist in Minneapolis, but they are a mere handful in a community of tens of thousands of Somalis who want nothing to do with extremist Islamism. [New York Times]