Anchored by Jake Tapper, The Lead airs at 4 p.m. ET on CNN.
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(CNN) – It is unclear just how many Americans have joined up with the militant terrorist group ISIS.
The fear is there is still a lot U.S. officials don't know, a federal law enforcement official told CNN, and it's an issue that's keeping officials buzzing around the clock.
U.S. officials are now closely monitoring Americans who have returned to the U.S. after fighting with ISIS, sources tell CNN.
But what makes this challenging is that not everyone who goes to Turkey or Syria goes to fight with terrorist groups, some of them are visiting family, or want to join humanitarian aid efforts. And there's a pretty high bar to put someone on a no fly list.
CNN's Pamela Brown reports.