Americans involved in Nairobi mall siege?

Are American terrorists involved in the ongoing siege in the Westgate mall in Nairobi, Kenya? It's a question now being investigated by the U.S. government.

That's because the organization behind the attack -- al-Shabab, an al Qaeda affiliate based in Somalia -- has long been known to have a strong presence in at least one American city: Minneapolis.

As Steve Kroft reported in May 2010 piece, Homegrown Terror, "By far the most troubling situation [of Americans aiding terrorist groups] is in Minneapolis, home to more than 50,000 refugees from Somalia. . . . Over the past two years, more than 20 young men have disappeared from their homes and turned up fighting in Somalia, with a terrorist group called al-Shabab."

In this excerpt from that 2010 report, Steve reports on al-Shabab's influence on young men in a Minneapolis neighborhood known as "Little Mogadishu."

One of al-Shabab's most successful American recruits is Omar Hammami, a U.S. citizen born to an American mother and a Syrian father. Growing up in Alabama, Hammami had no involvement in radicalism -- or even in the Muslim religion. He converted as a teenager and studied Islam under a local, controversial cleric.

Hammami became interested in Somalia several years later, after moving to Canada and marrying a Somali woman. Hammami and his new family moved to Egypt, but later ended up in Somalia as an al-Shabab fighter.

Hammami -- also known as Abu Mansoor Al-Amriki -- quickly rose through the organization's ranks, eventually becoming an al-Shabab spokesman and recruiter. He focused on recruiting young Americans with YouTube videos featuring radical rap songs.

Earlier this month, Hammami told Voice of America that he had left al-Shabab after nearly eight years with the group due to an argument with the group's leaders. It has since been reported that Hammami was killed just weeks ago by al-Shabab, yet those reports cannot be fully confirmed.

From Steve's 2010 piece "Homegrown Terror," here is more on Omar Hammami, one of al Shabab's most successful U.S. recruits.