(CNN) – Militant group ISIS released another gruesome video, showing the beheading of a second American, journalist Steven Sotloff.

The terrorist group has gained strongholds in eastern Syria and northeastern Iraq, and, according to a former CIA operative, ISIS cells have already infiltrated the U.S.

"The people who collect tactical intelligence on the ground, day-to-day – and this isn't Washington – but people collecting this stuff say they're here, ISIS is here, they're capable of striking," said CNN national security analyst and former CIA operative Bob Baer.



"They don't know what their plans and intentions are. But it's a definite concern," said Baer.

U.S. intelligence agents are keeping an eye on suspected ISIS militants who they believe have come across the Mexican border, or are American citizens that have come back from Syria, says Baer.

"They can't prove it. They're waiting to get enough intelligence to actually run them in. And then there's the unknown, of how many people have come back they're not even aware of," said Baer.

"The people who do this for a living are very alarmed," he says.

For more of our interview with former CIA operative Bob Baer, check out the video above.