The FBI served Google with a “national security letter” in early 2015, the company has revealed in this week’s transparency report, and obliged the company to keep the matter secret under a gag order until now.

National security letters allow the FBI to covertly subpoena data from companies without having to get a court order, so long as the requests are relevant to active investigations related to either terrorism or clandestine intelligence activities. Google did not offer details beyond the existence of the letter.

The report and the accompanying blog entry did, however offer other details showing growing government requests for data from Google, revealing FISA requests covering between 21,000 and 21,499 Google accounts, a significant increase from the previous year’s 16,000-16,499.

The government severely limits the ability of companies to make the exact details of their surveillance public, with gag orders keeping some actions secret outright, and even Google’s data on FISA requests is only allowed in “ranges” because they’re forbidden by law from giving exact numbers.