Just because there is no “credible evidence” of an Islamic State in Iraq and Syria (ISIS) plot against the U.S. doesn’t mean the extremist group isn’t planning one, said House Homeland Security Chairman Michael McCaul Michael Thomas McCaulHouse passes legislation to crack down on business with companies that utilize China's forced labor House Republicans blame Chinese cover-up for coronavirus pandemic Engel subpoenas US global media chief Michael Pack MORE (R-Texas) on Sunday.

“I think the biggest threat today is the idea that terrorists can communicate in dark space,” he said on CBS's "Face the Nation," referencing the myriad encrypted communications platforms that are widely available. “We can’t see what they’re saying.”

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McCaul acknowledged that the phrase “credible evidence” is “an old term of art.”

“I think you need to factor in that analysis that there may be plots under way,” he added, “that we just quite frankly can’t see.”

The issue of encrypted communications has been thrust into the spotlight following the recent terror attacks in Paris that killed around 130 people.

Officials have said it’s likely the ISIS followers behind the deadly strikes likely arranged their strategy via some type of encrypted communication, although no direct evidence has been presented to back up these suspicions.

“I think there’s strong indicators that they did,” McCaul said.

Encryption makes it more difficult for investigators to monitor digital data, including emails, certain types of text messages and social media exchanges.

“And that’s precisely why nothing was picked up,” McCaul said.

“The only rationale,” he added, “is that they were using these dark platforms and dark spaces to communicate, that even if we have a court order we can’t see.”

The Paris attacks have spurred a renewed debate on Capitol Hill about government access to digital data and encrypted communications.

Some lawmakers have even called for legislation that could require tech companies to give investigators guaranteed access to customer data.