By Mark Hosenball
WASHINGTON (Reuters) - The Justice Department has tapped a veteran prosecutor to probe the flow of foreign fighters including Americans who are joining Syria's rebels, U.S. officials said, in a sign of heightened alarm over the threat of radicalized militants returning home.
U.S. law enforcement and security agencies for months have expressed growing anxiety about a steady stream of Western militants, including Americans, heading to Syria. Most of the foreigners join up with the most extreme factions trying to oust Syrian President Bashar al-Assad, including Jabhat al-Nusra and the Islamic State of Iraq and the Levant.
U.S. and allied officials say their main fear is that veteran fighters, radicalized by their Syrian experience, will launch terror attacks once they return home. Authorities in Western Europe say they have uncovered plots by fighters returning from Syria.
The Justice Department prosecutor assigned to deal with the issue, Stephen Ponticello, works for the department's national security division.
In late March, the State Department also picked career diplomat Robert Bradtke to deal with the perceived threat. Bradtke will be the lead U.S. diplomat in "engaging foreign partners to prevent and interdict foreign extremist travel to Syria," a department spokeswoman said.
Until recently, U.S. intelligence officials had estimated around 7,000 foreigners since 2012 have joined about 23,000 anti-government rebels fighting with the most militant anti-Assad groups.
On Wednesday, however, State Department spokeswoman Jen Psaki said the estimate now stands at 9,000 foreign fighters who have traveled to Syria since the war started.
"We are engaged in a focused outreach effort with key partner governments regarding our shared concern over the flow of fighters to the Syrian conflict," Psaki said. She described U.S. allies in the Middle East and Europe as "gravely concerned" by the threat from their citizens traveling to Syria.
Psaki declined to estimate how many Americans have gone to Syria to fight. U.S. officials say that several dozen U.S. citizens or residents have traveled to Syria to fight with anti-government groups.
Hundreds of others from the United Kingdom, Belgium, Germany, France, Italy, Netherlands, Spain and other countries whose citizens are afforded easy entry to the United States also have fought in Syria, U.S. and European officials say.
U.S. authorities already have indicted a handful of individuals for alleged involvement in the Syrian conflict, often on charges of providing material support to militant groups. At least one U.S. citizen, Michigan native Nicole Mansfield, was killed in Syria last year, reportedly while fighting with anti-Assad forces.
"The issue of foreign fighters traveling to Syria, and their potential return to the U.S. and other Western countries, is of great concern to us," said a spokesman for the U.S. government's National Counterterrorism Center. "It's an area in which we are devoting significant time and resources."
(Editing by Warren Strobel and Paul Simao)