The Justice Department charged Hammami and 13 others, at least seven American, with funneling money and fighters to Somalia.
Hammami's internet videos, including one praising a fellow jihadist killed in an attack, are being used to recruit other Americans to the terrorist cause.
"We need more like him," Hammami says in a video. "So if you could encourage more of your children and more of your neighbors and anyone around you to send people like him to this Jihad it would be a great asset for us."
U.S. attorney general Eric Holder says, "We are seeing an increasing number of individuals, including U.S. citizens, who have become captivated by extremist ideology and have taken steps to carry out terrorist objectives either at home or abroad."
The twelve men indicted in the case are now on an FBI wanted poster. They are not in custody and are believed to be fighting with al Shabaab in Somalia.
Two women charged with raising money for the terror group by going door to door in Somali neighborhoods in Minnesota were arrested earlier today.
Minneapolis, with the nation's largest Somali population, is Ground Zero for al Shabaab recruitment. Terror experts say over the past four years some three dozen young men from that area have left the U.S. to join al Shabaab's ranks. Families there have turned to the FBI for help.
"It's their kids that have been recruited and in some cases ended up as casualties in Somalia. " says Minnesota U.S. Attorney B. Todd Jones. "So, parents are parents and they are very concerned,"
Al Shabaab, which has links to al Qaeda, has primarily focused on conflicts inside Somalia. But the group took credit for last month's twin bombings in Uganda which killed more than 70 World Cup soccer fans.
Now, al Shabaab is threatening to spread its violence to the West. And with so many al Shabaab fighters -- carrying U.S .passports -- the threat to this country cannot be dismissed.
CBS News national security analyst Juan Zarate says, "That has always been the worst case scenario, that American Somalis could be enlisted, trained and then deployed to hit sites in the United States."
U.S. officials say so far there is no evidence that al Shabaab is planning an attack against America. And the young men who have left for the fight have not yet tried to come home.