(CNN) Attorney General William Barr renewed the longstanding feud between law enforcement and tech companies over the issue of encryption on Tuesday, arguing that companies are jeopardizing authorities' ability to prevent major crimes.

Speaking at Fordham University's International Conference on Cyber Security, Barr said it was "dangerous" and "unacceptable" that tech companies' use of encryption to keep users' data safe from hackers and foreign governments is impeding law enforcement's ability to access communications after a warrant has been issued.

"By enabling dangerous criminals to cloak their communications and activities behind an essentially impenetrable digital shield, the deployment of warrant-proof encryption is already imposing huge costs on society," Barr said.

Citing several criminal cases where law enforcement was unable to access communications despite having a warrant, Barr said that encryption "seriously degrades the ability of law enforcement to detect and prevent crime before it occurs."

Barr cited the aftermath of the San Bernardino shooting of 2015, which led to a legal standoff with Apple after the FBI was unable to access the messages of an ISIS sympathizer, who killed 14 people, on a locked iPhone. Eventually the FBI purchased a third party walk-around to access the messages.

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