The documents reveal a New York FBI agent used a grand jury subpoena to demand subscriber information and communications log for the WhatsApp user. Even though the request came in 2014, there has been no record of it until now due to a gag order forbidding WhatsApp from disclosing the existence of the investigation to the subscriber or anyone else.

The use of such gag orders is controversial, and companies like Twitter and Yahoo have challenged them in court, arguing that they violate users’ due process and free speech rights.

The order to unseal the WhatsApp subpoena, issued by U.S. Magistrate Judge James Orenstein, does not include any explanation, but records show it came after a so-called “show cause” hearing in early January—suggesting WhatsApp may have asked for the gag order to be lifted.

A WhatsApp spokesperson, Matt Steinfeld, declined to comment on the case. John Marzulli, a spokesperson for the Justice Department in Brooklyn, said he could not comment “on any aspect of the subpoena.”

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The dispute over the WhatsApp investigation is significant because it suggests the Facebook-owned company is part of a broader pushback by the tech industry against secret demands for user information. Those demands can include gag orders on grand jury subpoenas or so-called “National Security Letters,” which are a form of subpoena that doesn’t require a judge.

The Justice Department and law enforcement agencies say such tactics are necessary to preserve the integrity of confidential investigations. Critics such as the ACLU, which joined an unsuccessful bid by Twitter and Yahoo to challenge recent gag orders, say the government should redact the orders rather than sealing up their very existence.

In the last year, the tech industry has gained some traction in its fight against gag orders. Last May, for instance, Facebook persuaded a judge to refuse a Justice Department demand because it used “boilerplate” language to justify 15 separate gag orders.

The unsealing of the WhatsApp subpoena, spotted by Verge reporter Russell Brandom, also comes at a time of mounting concern by consumers and activists about keeping their messages private.

In the case of WhatsApp, a news outlet in January reported the app contained a “backdoor,” but the report has since been discredited.

Overall, WhatsApp has a strong reputation for privacy, in part due to its use of so-called “end-to-end encryption.” The app also relies on cryptography from Open Whisper Systems, a company widely considered to be the gold standard for message privacy, and which stores almost no data about its users—as the FBI discovered in a court case in 2016.