The National Security Agency buys certain logs related to Americans’ domestic internet activities from commercial data brokers, according to an unclassified letter by the agency.

The letter, addressed to a Democratic senator and obtained by The New York Times, offered few details about the nature of the data other than to stress that it did not include the content of internet communications.

Still, the revelation is the latest disclosure to bring to the fore a legal gray zone: Intelligence and law enforcement agencies sometimes purchase potentially sensitive and revealing domestic data from brokers that would require a court order to acquire directly.

It comes as the Federal Trade Commission has started cracking down on companies that trade in personal location data that was gathered from smartphone apps and sold without people’s knowledge and consent about where it would end up and for what purpose it would be used.