The FBI issued a National Security Letter to Microsoft in 2013, which requested subscriber information about a single user account for one of the company's enterprise customers, according to documents unsealed on Thursday by a federal court in Seattle.
The letter had a nondisclosure provision that forbade Microsoft from disclosing the request to the company affected, which Microsoft concluded "was unlawful and violated our Constitutional right to free expression," wrote Brad Smith, the company's chief legal officer, in a blog post.
"It did so by hindering our practice of notifying enterprise customers when we receive legal orders related to their data," he wrote.
After Microsoft filed a petition challenging the NSL in U.S. District Court for the Western District of Washington, the "FBI withdrew its letter," Smith wrote.
But the unsealed documents showed the reason the FBI didn't challenge Microsoft's petition. The agency had obtained the information it sought directly from the Microsoft customer it had targeted in a way that maintained "the confidentiality of the investigation."
That reasoning would indicate the FBI might have fought Microsoft's petition if it hadn't achieved its aim of keeping the probe quiet.
Still, Smith considered it a victory for Microsoft. Although government requests for enterprise customer data are rare, "where we have received requests, we've succeeded in redirecting the government to obtain the information from the customer, or we have obtained permission from the customer to provide the data," he wrote.
"We're pleased with the outcome of this case, which validates our approach," Smith wrote.