Court filings have shown that the government was able to intercept phone calls and Messenger texts during its investigation, but three Messenger voice calls of interest were inaccessible. And while federal law requires telecommunications companies to provide law enforcement with access to phone calls when presented with a wiretap order, internet-based services aren't covered under that law. Facebook argued, apparently successfully, that Messenger fell within that exemption.
This move followed a number of attempts by the DOJ and FBI to gain access to mobile devices, asking companies like Apple to create backdoors through which they could acquire encrypted data. In one particularly notable instance, the FBI took Apple to court over its refusal to unlock an iPhone belonging to the San Bernardino shooter -- an action the DOJ later said was premature.
Sixteen suspected MS-13 members, including the individuals involved in the Messenger voice calls, were indicted earlier this month.