Yahoo has reportedly scanned hundreds of millions of email accounts at the behest of US intelligence or law enforcement.
The scans, reported by Reuters, allegedly selected incoming messages that contained a string of unknown characters.
Yahoo did not deny the report, saying only that it was a “law abiding company, and complies with the laws of the United States”.
According to the Tuesday report, Yahoo acceded to a 2015 government directive to give email access to the National Security Agency or the FBI. Reuters cited anonymous sources including two former employees and another person with knowledge of the events.
Yahoo continues to face questions about a breach in 2014 that compromised at least 500 million accounts.
The Department of Justice and the FBI did not immediately respond to requests for comment.