US government investigators have lost a case to force Facebook to wiretap calls made over its Messenger app.

A joint federal and state law enforcement effort investigating the MS-13 gang had pushed a district court to hold the social networking giant in contempt of court for refusing to permit real-time listening in on voice calls.

According to sources speaking to Reuters, the judge later ruled in Facebook’s favor — although, because the case remains under seal, it’s not known for what reason.

The case, filed in a Fresno, Calif. district court, centers on alleged gang members accused of murder and other crimes. The government had been pushing to prosecute 16 suspected gang members, but are said to have leaned on Facebook to obtain further evidence.

Reuters said that an affidavit submitted by an FBI agent said that “there is no practical method available by which law enforcement can monitor” calls on Facebook Messenger . Although Facebook-owned WhatsApp uses end-to-end encryption to prevent eavesdroppers, not even the company can listen in — which law enforcement have long claimed that this hinders investigations.

But Facebook Messenger doesn’t end-to-end encrypt voice calls, making real-time listening in on calls possible.

Although phone companies and telcos are required under US law to allow police and federal agencies access to real-time phone calls with a court-signed wiretap order, internet companies like Facebook fall outside the scope of the law.

Privacy advocates saw this case as a way to remove that exemption, accusing the government of trying to backdoor the encrypted app, just two years after the FBI sued Apple over a similar request to break into the encrypted iPhone belonging to San Bernardino shooter Syed Farook.

FBI declined to comment. Facebook did not respond to a request for comment.