They could strike the United States. That grim assessment is the first time the FBI director or any other senior law enforcement or intelligence official has stated on the record that the Al Qaeda-linked group al-Shabaab is no longer content to strike within the East African nation of Somalia.

During a hearing on Capitol Hill on Wednesday, FBI Director Robert Mueller was asked if members of al-Shabaab, which translates as "mujahideen youth," would send American recruits back to the U.S. to launch attacks.

"I would think that we have seen some information that the leaders would like to undertake operations outside of Somalia," Mueller told the Senate Homeland Security Committee.

Mueller said he is "absolutely" concerned that Americans who traveled to Somalia to train as terrorists  would have U.S. legal status and would therefore be able to return to the United States and carry out attacks.

A U.S. counterterrorism official told FOX News that al-Shabaab has exploded since 2006, and it is becoming a full-fledged Al Qaeda affiliate, similar to Al Qaeda in Iraq. Initially the group's militias fought against the Somali government and Ethopian forces who are against an Islamic state in East Africa, the official said, but now the group's focus is turning toward the establishment of a "caliphate" or broad Islamic state not limited to Africa.

In fact, in an exclusive briefing on Thursday, a U.S. counterterrorism official told FOX News that al-Shabaab's leadership is bona fide Al Qaeda. In many cases, the operatives who guide the group in East Africa have pledged allegiance to Usama Bin Laden in person, and in some cases the Al Qaeda leader has entrusted them with missions outside of the Afghanistan-Pakistan border, according to the official.

Training camps are now well-established in Somalia, and as many as 1,100 foreign fighters have joined the al-Shabaab movement, the president of Somalia's transitional government told a crowd gathered at the Center for Strategic and International Studies in Washington on Wednesday.

A U.S. counterterrorism official said there are other troubling developments. A recent videotape from al-Shabaab pledged a formal allegiance to bin Laden, and the intelligence community is now monitoring jihadist Web sites to see if bin Laden will respond and "officially" bless their relationship, the official said.

In addition, the use of simultaneous attacks and suicide bombings -- the signature of al Qaeda -- has now taken hold in Somalia.

In October last year, a Minneapolis man, Shirwa Ahmed, became the first known American suicide bomber, according to the FBI. Ahmed, who travelled from Minnesota to the training camps of al-Shabaab, is one of nearly two dozen men from the United States who authorities believe joined the group.

The FBI is currently investigating another man, an 18-year-old from Seattle, who may be the second documented case of an American suicide bomber. Investigators are comparing the man's DNA with remains from an attack in the Somali capital last month.

In early September, FOX News exclusively identified the American mouthpiece for al-Shabaab as a native of Daphne, Ala. Born Omar Hammami, the big-eared and bright-eyed teenager now goes by the name Abu Mansour al-Amriki, or simply "The American."

After FOX News uncovered al-Amriki's identify, his family told a local TV station in Mobile, Ala., that their son traveled to Somalia with his wife but lost his passport there, where he became indoctrinated by al-Shabaab. One source within Mobile's Muslim community said that, after dropping out of college, Hamammi travelled to Toronto, where he eventually married a Somali woman and became indoctrinated there by "angry" individuals.

Both Hammami's family and the source within Mobile's Muslim community said Hammami has not spoken to his family in years.

Catherine Herridge is an award-winning Chief Intelligence correspondent for FOX News Channel (FNC) based in Washington, D.C. She covers intelligence, the Justice Department and the Department of Homeland Security. Herridge joined FNC in 1996 as a London-based correspondent.