The Trump administration said on Thursday that it had brought home and would prosecute a US national suspected of fighting alongside ISIS in Syria.
The Pentagon said the citizen was held by the Syrian Democratic Forces as a suspected member of ISIS.
Tens of thousands of foreign fighters from across the world travelled to Syria to join ISIS after it declared a global caliphate there in 2014.
As the militants in the past five years have lost the territory they once controlled, a couple of thousand are still in detention.
The identity of the person was not immediately disclosed and it was not clear where in the US they had been taken.
The Justice Department had no comment and no charges were immediately laid.
US officials have been encouraging other nations to take home citizens who fought in Syria with ISIS.
The Justice Department considers prosecutions in American courts as an example that could be set for other countries.
But European nations have shown reluctance to repatriate their citizens who are in custody.
"If you can prosecute, then that's the thing you do," Assistant Attorney General John Demers, the Justice Department's top national security official, said in June at the EU and Foreign Policy Defence Forum.
Mr Demers said the US, hoping to encourage other countries to take responsibility for their citizens, had put together packages on people who travelled overseas to join ISIS and shared the information with other countries for purposes of potential prosecution.
Nathan Sales, the State Department's counter-terrorism co-ordinator, estimated that about 2,000 people from around the world were being held by Syrian Democratic Forces.
"Now we have to make sure that they're never able to return to the battlefield," Mr Sales said.
"So this is in part a law-enforcement problem. It's a military problem. It's an intelligence problem. It's a border security problem.
"It's a really knotty problem that sort of captures all the various different aspects of national security and counter-terrorism in one tidy package."
US authorities routinely bring charges under a statute that makes it a crime to provide any sort of material support to a foreign terrorist organisation.
