(CNSNews.com) - "I think we are in a dangerous time," Attorney General Eric Holder told ABC News in an interview that aired on Sunday.
Holder said he's worried about the estimated 7,000 Americans and Europeans now fighting in Syria, who will return home, intending to wage terror attacks.
"In some ways, it's more frightening than anything I think I've seen as attorney general. You know, 9/11 was something that kind of come out of the blue. This is a situation that we can see developing, and the potential that I see coming out, the negative potential I see coming out of the facts in Syria and Iraq now are quite concerning."
Holder, asked how many Americans have gone into Syria and already made their way home, said there are "dozens of investigations underway." "The FBI is on top of these," he said.
He also expressed concern about Yemeni bomb-makers traveling to Syria: "That's a deadly combination, where you have people who have the technical know-how, along with the people who have this kind of fervor to give their lives in support of a cause that is directed at the United States and directed at its allies. And it's something that gives us really extreme, extreme concern."
He said the U.S. is "constantly monitoring" what known bomb-makers are doing, which explains the new security measures at some overseas airports, where passengers may be asked to turn on their electronic devices at security checkpoints.
"This is not a test," Holder said. "We're doing something in reaction to things that we have detected."
Another looming threat is ISIS, the Islamic State of Iraq and Syria, which is now taking over large portions of Iraq.
"If they are able to consolidate their gains in that area (Iraq and Syria), I think it's just a matter of time before they start looking outward and start looking at the West and at the United States in particular. So this is something that we have to get on top of and get on top of now," Holder said.
And then there are the homegrown radicals: "These lone wolves, these homegrown violent extremists are people who keep me up at night as well, trying to monitor them, trying to anticipate what it is that they are going to do. And, you know, the experience that we had in Boston is instructive. It only takes one or two people to really do something horrific."