"With all these encryption software programs, we can’t penetrate into certain conversations, and we’re dealing with this gigantic black hole, a dark zone where there are just so many dangerous things going on," Francois Molins said on "60 Minutes."
Molins said he runs into a texting app often used by ISIS called Telegram, which offers advanced encryption.
He said he's come up against the app "very often" in investigations.
"Telegram, we can't penetrate, we can't get into it," he said.
Rob Wainwright, head of Europol, said on "60 Minutes" that encryption has been a problem in most investigations.
"I mean, across the tens of thousands of investigations that Europol is supporting every year on terrorism and serious crime, at least three quarters of them have encryption at the heart of the challenge that law enforcement face," Wainwright said.
He said from what he sees, "encryption also played in a role in that part" in the Nov. 13 attacks but said that's something they are "digging into much deeper at the moment." He said he can't share details about the investigation.
At the end of the segment, when Molins was asked about whether it's more important to shut down terrorism or preserve privacy, he said he thinks there are limits to freedom of privacy.
"Freedom doesn't mean you can just do anything and everything you want. And there's a duty of institutions — police and judicial — to ensure security. You can't have freedom without security," he said.
The segment aired amid a fierce debate in the U.S. between Apple and the FBI over a locked iPhone.