Outgoing FBI Director Robert Mueller is concerned about growing domestic terror threats, and says he is particularly worried about Americans traveling to Syria and coming back with intentions “to undertake an attack upon the homeland.”

Speaking to reporters on Thursday before he departs the FBI next month, Mueller said terrorist threats are now originating in countries such as Tunisia, Libya, Mali, Algeria, Syria and Egypt. Syria, in particular, had been recruiting fighters from around the world, including Americans, he noted.

“Every one of these countries now has cadres of individuals who you would put in the category of extremists, violent extremists that will present threats down the road, not only threats to us here in the United States, but also threats to Americans overseas,” Mueller said.

The United Nations estimates that at least 100,000 people have died since the Syrian uprising began in 2011. More Westerners have traveled to Syria to participate in the conflict than the number of those who fought in Iraq, Afghanistan, Somalia or Yemen. Matthew G. Olsen, the director of the National Counterterrorism Center, last month called Syria “the predominant jihadist battlefield in the world.”

And Mueller, who served as FBI director for 12 years, is concerned that Syria could breed a radicalized class of American terrorists that may conduct further attacks in the US.

“[When] you have individuals traveling to those venues, you are concerned [first] about the associations they will make, and secondly about the expertise they will develop and whether or not they will utilize those associations, utilize that expertise, to undertake an attack upon the homeland,” Mueller told ABC News’ Pierre Thomas in an exclusive interview.

Mueller said FBI officials are concerned about and monitoring the flow of American fighters into Syria.

But Mueller is not just concerned about American fighters who might be radicalized in Syria; the outgoing FBI director said he is also troubled about the possibility that terrorists might use biological weapons of mass destruction, or that domestic attacks could be conducted by American citizens that he described as “lone wolf” suspects.

He pointed to the rising danger of homegrown terrorism, such as the attacks at Fort Hood and the Boston Marathon. Although Boston bombing suspect Tamerlan Tsarnaev came “to our attention” before the attack occurred, Mueller said the FBI could do nothing more beyond conducting its investigation. Unless there is stark evidence that a terrorism suspect has plans to conduct an attack, federal authorities are not able to take any action against a suspect.

“In the future, we will have a case where the person has been on our radar screen, but we do not have the evidence and there is no outward evidentiary capability to bring the person into the criminal justice system,” Mueller warned, expressing fear that further domestic terrorist attacks might occur in the future. “That is going to happen.”

Mueller’s term as FBI director expires on Sept. 4. He will be succeeded by former US Deputy Attorney General James Comey.



