(MSNBC) — The suicide bombing last weekend in Mogadishu — allegedly by a Somali American from Minnesota — has highlighted the important role played by U.S. citizens in the operations of al-Shabab, the Islamic terrorist organization battling the government in the war-torn east African nation.

If it is confirmed that Abdisalan Hussein Ali was one of two suicide bombers who attacked an African Union base, killing themselves and eight others, it will have been the third suicide bombing carried out in Somalia by Americans since 2008.

And U.S. officials tell NBC News that at least two members of the al-Shabab hierarchy are American-born, 20-something college dropouts, one of whom may be in the group’s “inner circle.”

U.S. officials and counterterrorism analysts estimate there are at least 40 Americans fighting with al-Shabab in Somalia, as well as another 200 with passports that would permit them to enter the U.S. without a visa.

Many of the al-Shabab soldiers are Somali-Americans, many of them from the Minneapolis area, like Ali. The two leaders are not. They are Arab-Americans who traveled to Somalia in the latter part of the last decade and began rising in the ranks of the al-Qaida-linked terrorist group.

One — San Diego-native Jehad Marwan Mustapha — is believed to be part of the group’s senior leadership. The other, Omar Hammami, is a unit commander.

Hammami’s role has long been known. The 27-year-old from a suburb of Mobile, Ala., was profiled in the New York Times Sunday Magazine last year and has appeared in a number of al-Shabab videos, including one where he rapped an English-language recruiting pitch.

Hammami, whose father is Syrian, joined al-Shabab in late 2006 and took the name “Abu Mansoor al-Amriki,” or Abu Mansoor the American.

Mustapha, 29, is less well-known and has a less public role in al-Shabab, but is likely more influential in the terrorist group, according U.S. officials and Evan Kohlmann, an NBC News terrorism analyst.

“Though his name is perhaps lesser known than that of American national Omar Hammami, Jehad Mustapha is nonetheless reputed to be among the very top leaders of the foreign jihadists fighting alongside Shabab al-Mujahideen in Somalia under the banner of al-Qaida,” Kohlmann said.

One U.S. counterterrorism official, who like the others in this article spoke on condition of anonymity, referred to Mustapha as a “pretty bad dude” who has been with al-Shabab for “several years . . . long enough to be a significant commander . . . a senior player in the organization.”