Microsoft has admitted the FBI is using secretive National Security Letters to spy on its customers without obtaining a warrant. The news follows a similar announcement by Google earlier this month, which marked the first time a U.S. company has admitted receiving the secretive requests for user data. The letters come with a gag order that bars the recipient from talking about them. Earlier this month, a federal judge ordered the government to stop issuing the letters, ruling they are unconstitutional and giving the Obama administration 90 days to appeal.