Federal law enforcement officials are deeply concerned about that threat, issuing some stark warnings. ABC's Pierre Thomas has the latest. Reporter: Islamic radicals. Some more ruthless than Al qae... See More
Federal law enforcement officials are deeply concerned about that threat, issuing some stark warnings. ABC's Pierre Thomas has the latest. Reporter: Islamic radicals. Some more ruthless than Al qae Qaeda, overrunning cities in Iraq. In Syria, training, learning, killing. Two conflicts joined by extremists from around the world. Some of them, like this young man, are Americans. Some claims they turned him into a suicide truck bomber, who claim he did this in Syria. Which could become a clear and present danger for the homeland here. The next 9/11 is in the making. Reporter: In an exclusive ABC news interview, the FBI director, the homeland security adviser jeh Johnson issued warnings. At least 70 Americans have amentaledly joined the thousands of foreign fighters in Syria. With my european counterparts we have been working together on sharing information and tracking these information. Reporter: Some of those potential terrorists have already made their way back to the U.S. Can you give us a sense of how active you are that you're keeping an eye on those people? Very active. It's offering an opportunity for thousands of foreign fighters to get training in the worst arts of terrorism. Reporter: Just this week. We're determined not to allow a line from today's Syria to be drawn to future 9/11s. Reporter: The radicals are flowing back and forth over the border between Syria and Iraq. They have done more than take over cities in Iraq, they have captured military equipment and money. They can use that money to buy equipment and move people around. Reporter: Including possibly to the U.S. Homeland. For "This week," Pierre Thomas, ABC news, Washington. With us now, congressman Michael Mccaul, the chairman of the homeland security committee, and Martha is back with us from Baghdad. Mr. Chairman, let me start with you, you issued a statement saying the president should take immediate action in Iraq, what do you think he should do? This is a crisis. It calls for response. I talked to ambassador crocker yesterday, he said that this is the greatest threat, national security threat since 9/11. Al Qaeda owns more territory and more resources. And what's happening in Iraq right now is really chaotic. I think what needs to be done. The white house called me and asked me for my advice, I would call the top team of commanders and diplomats who run this war and get their advice moving forward. I tell you, it's the president's inability to negotiate in Iraq, which has put us in this situation today. I'm asking you the immediate action, you're not just calling for him to get advice, should he take military action right now? I think he needs to look at his options. In talking to the ambassador, he believes that a diplomatic political solution between the Sunni, shia and kurds, need to take place now. Kerry and Hagel need to be in the region now. They need to get a regional strategy together with our allies to deal with this situation. Without their cooperation against the extremists this isn't going to happen. They're not going to do it on their own. They need us to lead them and we're not leading right now as a nation. Now, when the president addressed this issued on Friday, he twice used the word eventually, when talking about the threat this group would pose to U.S. Interests, how far down the line until these pose a threat to the U.S.? This Al Qaeda faction, Isis, in Syria and now Iraq, it's so extreme that Al Qaeda has denounced them. That's how dangerous they are. This isn't a down the road prospect. I would argue that not only is the threat great in Iraq and Syria, but so to the homeland. Because we have a lot of individuals over there with legal travel documents that are trained, and these are the vicious, this is the worst of the worst, if they get back into the United States or western Europe, I see that as the biggest threat today. So, Martha, you're there on the ground, we see that black flag of Al Qaeda flying over Iraqi cities, what is your sense on this group's vision and how much does attacking American interests, American targets fit into their plans? I think there's no question this is a group that is anti-american, and if they could attack America today, they probably would, I don't think they're ready for that. But look at the equipment they're getting and the other people joining them in the fight. It's total chaos over here right now. I can't stress that enough. Here in Baghdad, people are truly afraid. They think they're going to be attacked. When you look at the western interests around here, there are a lot of Americans here, there are a lot of westerners here and you can be sure they would try to target them. And you know what, you think about this, Mr. Chairman, the reports out of nineveh province, this group has seized some 4$400 million money out of Iraqi banks, this could be a much well-funded Al Qaeda we have seen. Yes. They have our weapons now. It's a highly good recruiting mechanism for them. What we really have, is, Jon, we have a vacuum, the vacuum is being filled by this extreme Al Qaeda faction. What concerns me is that Iran is moving in. But don't think for a minute that Iran is not looking at this vacuum as a potential power grab. Any unease if the U.S. Acts we'll be acting on the side of the Iranians. That's why we have to be very careful with any military strategy. Diplomatically, we got to bring Sunni, shia and kurds together. Failed to engage in the shia tribal leaders to stop this and now the blowback is happening. But we need to stop this. The action needs to be now, not two weeks down the road. The president should come back to the white house. Get a team of experts together. Very quickly, you heard Lindsey graham say, we may be seeing another 9/11 in the making? Do you believe with that? I believe it's one of the biggest threats we have seen. In fact, I talked to a mother in my district, just saying, you know, my son died in fallujah, he believed in what he was doing and I just want it to count for something and what I'm seeing today makes me sick. I don't want to have to question
This transcript has been automatically generated and may not be 100% accurate.