Last December, al-Shabab slapped Hammami publicly in an Internet statement, saying his video releases are the result of personal grievances that stem from a "narcissistic pursuit of fame."
Militants in Somalia have long hosted foreign fighters in the country. U.S. officials say that al-Shabab, which has been around since about 2006, counts several hundred foreign fighters among its ranks, including several dozen Somali-Americans from Minnesota.
Al-Shabab and al-Qaida announced formal merger in February 2012, but the Somali militant group maintained a reputation as being hostile to foreign fighters.
"Hammami brought a lot of unwelcome outside scrutiny on Shabab from the international jihadist community. His story will likely be a case study on what can go wrong when Westerners join jihadist movements," Berger said.