The U.S. intelligence community has grown increasingly concerned in recent months about the flow of European and American nationals to fight with al Qaeda-linked affiliates in Syria’s civil war.

While the number of Americans who have traveled to join the jihad in Syria is low, some members of the U.S. intelligence community worry that fighters who receive training and battlefield experience in Syria could return to the West with skills honed by fighting in the country’s civil war.

“At some point all of these people from Europe are going home. All the folks there from all over the world, including the United States, will be coming home if they do not meet their end on the battlefield,” said Rep. Mike Rogers, the Republican chairman of the House Permanent Select Committee on Intelligence, on Tuesday.

U.S. intelligence estimates differ on how many Americans have joined the fight with al Qaeda. Some officials say the number could be as high as 60 U.S. citizens, while others put it much lower, at 10. Hundreds of European nationals have joined al Qaeda-linked groups since the civil war began more than two years ago, say U.S. intelligence officials.

Aaron Zelin, a senior fellow at the Washington Institute for Near East Policy and an expert on al Qaeda affiliates in the Middle East and Africa, echoed those estimates. “I have seen anywhere from 10 to 20 Americans based on jihadi forums and Arabic and Western media who have joined extremists in Syria,” he said. “In terms of the European numbers, it’s definitely in the hundreds. It’s closer to 700 or 1,000, according to European officials I have spoken with.”

Fred Hof, who served in 2012 as the State Department’s special adviser for the transition in Syria, said: “I have seen plenty of reporting of foreign fighters coming from around the world. Obviously there are plenty from Europe, there may be [some of] Tunisian or Algerian ethnic background.”

Rogers told The Daily Beast that the civil war in Syria is a “jihadist magnet.” He added, “From Europe, from northern Africa and other places in the Middle East, and in some rare instances from the United States of America, they are getting training and further radicalization in Syria.” The U.S. government is aware of conversations between al Qaeda’s core leadership and al Qaeda affiliates in Syria and that parts of eastern Syria would make a “great safe haven,” he said. Other U.S. intelligence officials confirmed his account to The Daily Beast.

The pattern of volunteers traveling long distances for a chance to fight with al Qaeda is reminiscent of the war in Afghanistan against Soviet occupation in the 1980s, when Osama bin Laden raised money and helped funnel foreign fighters into that country to fight with the Islamic warriors supported by Pakistan’s military intelligence service. The Afghanistan war predated the formation of al Qaeda, but as in Syria today, the jihadists were only one faction of the overall opposition to the Soviet-aligned government in Kabul.

More recently, the U.S. war in Iraq and the civil war in Somalia have served as rallying points for the world’s Islamic extremists. Since the middle of the last decade, al Qaeda’s affiliate in Somalia, al-Shabaab, has tried to recruit fighters from the Somali-American community in Minneapolis.

One problem in tracking the flow of Americans and Europeans into Syria is the difficulty of assessing the hundreds of cells that comprise the opposition. At the Aspen Security Forum in July, the Defense Intelligence Agency’s deputy director, David Shedd, estimated the number of Syrian rebel groups at 1,200. The two main al Qaeda-related groups among the rebels are al-Nusra and the Islamic State of Iraq and the Levant (ISIL). ISIL, an extension of the al Qaeda affiliate that fought U.S. soldiers and the Iraqi state in the last decade, launched a wave of assassinations this summer and has struggled for leadership of the jihadist wing of the Syrian opposition with al-Nusra. There are also other extremist militias that share the ideology of al Qaeda but not any formal connections to the organization.

In Aspen in July, Shedd warned, “If left unchecked, I am very concerned the most radical elements will take over larger segments of those 1,200 groups.” He added that he believed most of the fighters opposed to Syrian regime were interested in local issues and not the wider goals of al Qaeda.

Nonetheless, Shedd said al Qaeda’s leader, Ayman al-Zawahri, had provided “spiritual direction to the fight” for al Qaeda’s affiliates in Syria. He also acknowledged that fighters from al Qaeda’s core have migrated to Syria to help wage a war against the Syrian regime.

Some Americans who have joined the jihad in Syria have already received attention from U.S. authorities. In April, a federal court charged Eric Harroun, a former U.S. Army private from Arizona, with firing a rocket-propelled grenade while fighting with al-Nusra. In May the Detroit Free Press reported that FBI agents were interviewing the family of a convert to Islam named Nicole Lynn Mansfield. Her family told the paper that she was killed fighting with the Syrian opposition.