(CNN) -- A possible threat posed by thousands of westerners who have been drawn to Syria by the ongoing civil war has become a top worry among counter-terrorism officials in Europe and the United States, FBI Director James Comey said Friday.
Dozens of U.S. residents have gone to Syria, made plans or attempted to go, Comey told reporters. Of equal concern, he said, were citizens of European and other countries who can easily enter the United States because of visa-waiver agreements.
He drew comparisons to the thousands of foreigners who received military training in Afghanistan in the 1980s and 1990s, after first being drawn with U.S. encouragement to fight communism and then to join militant factions during the Afghan civil war.
Some of those fighters became al Qaeda operatives, he noted, drawing a line from the Afghan war to the September 11, 2001, terror attacks in the United States.
"There's going to be a diaspora out of Syria," Comey said. "And we are determined not to let lines be drawn from Syria today to a future 9/11."
The comments came in a meeting with reporters at FBI headquarters that covered a number of topics.
Comey said his agency was working with police around the country to try to help them respond to, and possibly thwart, mass shootings.
He said the FBI's Behavioral Analysis Unit is now providing advice to police in cases where they discover a possible plot in the planning stage.
In the past week, he said, with the FBI's help, one police agency was able to intervene and get a person to mental health treatment, possibly stopping an attack.
Comey declined to provide more details about the incident.
Comey also said police agencies around the country are now getting FBI training to do cyber investigations.
As more crime moves from the street to online, more victims of theft could be helped by local police who are trained to do such investigations.
That would allow the FBI to better use its limited resources, he said.
"Our strategy is for the FBI to try and focus its resources on the national security intrusions, and the largest most sophisticated criminal operations," Comey said.