SAN FRANCISCO, Nov. 28 (Xinhua) -- A U.S. rights advocacy group on Wednesday filed a motion with a federal court in California to disclose how the U.S. government had forced Facebook to wiretap encrypted voice conversations of users on Facebook Messenger.

The American Civil Liberties Union (ACLU), a group advocating rights and civil liberties, wants to ask the court to reveal details about the federal government's failed attempt to secretly recode the Facebook-owned Messenger app and listen to the conversations of Messenger users.

"A court decision with the potential to affect the privacy rights of millions of people should be public," ACLU tweeted Wednesday.

"The public has a right to know the legal reasoning that decided this case - namely, what authority the Justice Department thought it had to force Facebook to undermine its security infrastructure," said the group in a statement.

ACLU quoted earlier media reports as saying that the Justice Department sought an order from a federal court to force Facebook to wiretap encrypted voice conversations of Messenger users as part of an investigation into the MS-13 gang, an international criminal gang that originated in Los Angeles, California, in the 1980s.

Facebook reportedly refused on arguments that rewriting the Messenger app's code would undermine its security infrastructure for all users. The government sued Facebook in court, but the case later collapsed.

"Facebook may have won this time, but if the government tries to force another service to undermine its security features and that service wants to fight back, it won't be able to rely on the court's reasoning so long as the opinion remains under wraps," ACLU said.

It called for greater transparency as the "government has a track record of hiding from public oversight."

More than 1.3 billion people are using Facebook's Messenger around the world.