Another video, originally obtained by a Christian news agency, shows a recent convert to Christianity being beheaded for violating the group's interpretation of Islamic law, or Sharia.
Former fighters now living in Kenya told ABC News that many young Somali men have no choice but to join al Shabaab.
"They say you are either supporting the Ethiopian[s] or you support us. Or if not, we'll kill you," said one former fighter. "These are the most dangerous people."
There's been no real Somali government since 1992, and only the presence of Ethiopian troops and tanks and U.S. aid has kept the transitional government in Mogadishu from collapse. Counterterrorism experts say that the deteriorating situation will force the incoming Obama administration to pay attention to Somalia.
"But Obama will not have any more options than the current administration has, and they are very few," said Clarke.