U.S.-backed Syrian forces on edge amid concern ISIS could try to stage a comeback on heels of Assad's ouster

Eastern Syria — CBS News was among the first news outlets to speak on Thursday with Travis Timmerman, an American who was feared dead by family and friends, days after he was freed from a notorious prison in Syria. He said he had spent seven months jailed by the regime of now-ousted dictator Bashar al-Assad before rebels broke down his cell door.

But he was just one of the many thousands of people locked up over half a century of iron-fisted rule by Assad, and his father before him. Many remain missing, and the rebel forces, along with the families of those who've disappeared without a trace, have mounted a herculean effort since Assad fled to Russia on Sunday to find those who vanished.

But there's one group of prisoners that Syria's still-evolving, rebel-led leadership wants to keep behind bars. Five years ago, as U.S.-backed forces wrested control of land held for years by ISIS, CBS News visited a prison where the members of the terrorist group were being held. This week, CBS News returned to the prison in eastern Syria. Guards said it was still holding thousands of ISIS militants, but they wouldn't say exactly how many.