Neither US prosecutors nor Facebook have commented publicly about the Messenger case

Two civil rights groups asked a federal judge on Wednesday to release documents describing a secret U.S. Department of Justice effort to force Facebook Inc to decrypt conversations between users.

The American Civil Liberties Union (ACLU) and the Electronic Frontier Foundation (EFF) filed a joint motion in U.S. District Court in Fresno, California, the ACLU said.

The Justice Department's bid was part of a joint federal and state probe of the activities of the MS-13 gang in Fresno, revolving around the end-to-end encryption Facebook uses to protect calls on its Messenger service from interception. End-to-end encryption means only the communicating users can read messages.

Neither U.S. prosecutors nor Facebook have commented publicly about the Messenger case due to a gag order. But Reuters reported in September that investigators failed in a courtroom effort to force Facebook to wiretap Messenger voice calls.

In a draft of the motion shown to Reuters by the ACLU, the civil rights group argues that the public's right to know the state of the law on encryption outweighs any reason the Justice Department might have for protecting a criminal probe or law-enforcement method, especially since the legal fight has been resolved.

"Although it's possible that other courts have confronted similar issues in secret, the Eastern District of California may be the first court to rule on whether the federal government can force a private social media company to undermine its own security architecture to aid a criminal investigation," the motion says.

The motion seeks the release of the government's arguments and any ruling accepting or rejecting each of those arguments. It says the court could redact information about individuals that could hurt a criminal case.

Telecommunications companies are required to give police access to calls under federal law, but many apps that rely solely on internet infrastructure are exempt. Facebook contended Messenger was covered by that exemption, sources told Reuters.