Biggest threat to US homeland? 'More work to do' securing air travel.

The most significant threat to the US homeland is the risk that a terrorist group would bring down a civilian airliner.

That is the assessment of Rep. Adam Schiff (D) of California, the ranking member of the House Permanent Select Committee on Intelligence. He is one of a handful of members of Congress who receive the most detailed intelligence briefings.

At a Monitor-hosted breakfast for reporters on Tuesday, Representative Schiff voiced skepticism about claims made by computer expert Chris Roberts. In a March interview with the Federal Bureau of Investigation, Mr. Roberts allegedly said that he had tampered with an under-seat box on a plane and used it to gain access to in-flight systems and from there to the aircraft’s avionics.

“Obviously, we are looking into those allegations with a lot of sense of urgency,” Schiff said. “We are trying to find out was that even possible on the aircraft.”

Even if Roberts did not hack the aircraft, Schiff added, “is that nonetheless a vulnerability?”

Schiff said the most likely form of terrorist attack is from “self-radicalized” individuals taking part in “one-off” acts. “People that may use a vehicle or a hatchet or a gun to commit an act of violence,” he said.

But such random acts of violence do not constitute the biggest threats to national security, Schiff said. “The more significant threats we continue to face are to our aircraft and that would be predominantly by groups like AQAP that have been really determined to put explosives on our aircraft,” he said. 

AQAP, or Al Qaeda in the Arabian Peninsula, is a Sunni extremist group based in Yemen that has been behind numerous high-profile terrorist attacks including an attempt to bring down a Northwest Airlines flight on Christmas Day in 2009.

Schiff said Tuesday that the likelihood of a coordinated mass attack like that on Sept. 11 “has been greatly diminished,” but added that the nation has “a lot more work to do in terms of securing our airports, aircraft, and transportation systems.”

Meanwhile, foreign fighters are streaming into Syria and Iraq in unprecedented numbers to join the Islamic State and other extremist groups, the Associated Press reports, citing US intelligence officials. One concern is that some foreign fighters will return to their homes in Europe and the US to mount terrorist attacks.

"In the category of growing threats, you have to include all of the foreign fighters who have started coming home," Schiff said. "It is already a challenge for us in the United States, and that is only going to increase."