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Foreign Fighter and Lone Wolf Threats Growing, Experts Warn

U.S. intelligence officials told Congress Wednesday that the flow of potential terrorists to Syria is greater than it's been to anywhere in decades.

U.S. intelligence officials told Congress Wednesday that the flow of potential terrorists to Syria is greater than it's been to anywhere in decades — while a group that tracks domestic terrorism urged the Obama administration not to focus only on the threat of radical jihadism.

"The rate of foreign fighter travel that we've seen in recent years is unprecedented," said Nicholas Rasmussen of the National Counterterrorism Center. He said it's more than the potential terror travel to Afghanistan, Pakistan, Iraq, Yemen or Somalia.

"It exceeds the rate of travel to those conflict zones that we've seen at any point in the last 20 years."Western intelligence agencies say more than 20,000 foreign fighters have traveled to Syria from over 90 different countries -- at least 3,400 of them from western countries. From the US, the FBI says, more than 150 people have either gone to Syria or tried to go there.

"We are increasing our efforts to track those who enter and leave Syria and may later seek to travel to the United States," said Francis Taylor of the Department of Homeland Security.

Airline passengers coming to America from countries whose residents do not need a visa to enter the U.S. are getting increased attention, he said, with the U.S. requesting additional data on those travelers before they board flights.

From left: Homeland Security Undersecretary for Intelligence and Analysis Francis Taylor speaks, watched by National Counterterrorism Center Director Nicholas Rasmussen, and FBI Counterrorism Division Director Michael Steinback during a House Homeland Security Committee hearing on "Countering Violent Islamist Extremism: The Urgent Threat of Foreign Fighters and Homegrown Terror" on February 11, 2015 in the Cannon House Office Building on Capitol Hill in Washington, DC.MANDEL NGAN / AFP - Getty Images

The FBI's Michael Steinbach said every person who returns from the U.S. from Syria is subject to an investigation because of the threat from ISIS, which the federal government calls ISIL.

"We seek to determine the root cause of their travel, what they did in Syria, and then ultimately, if it was in support of a foreign terrorist organization such as ISIL, we look for prosecution or some other disruption."

But, Rasmussen said, "the information we have about foreign fighters traveling into the conflict zone stops when they get there.

"We don't have as much insight into what actually happens when they're on the ground in Syria. And that's a gap we're trying to close."

Also Wednesday the Southern Poverty Law Center said its analysis of recent domestic terror-related incidents shows that the federal government should not lose sight of the threat of violent extremism that is not the result of jihadist radicals.

In the past six years, the group said, "more people have been killed in America by non-Islamic domestic terrorists than Muslim jihadists."

Some are motivated by anti-government ideology, some by pure hate, the group said.

And nearly all the attacks, according to its report, were planned or carried out by a single person.

It found that 74 percent of the 60 incidents the group examined from 2009 to 2014 were the work of a single person. "A total of 90 percent of the incidents were the work of just one or two."