Yahoo this week publicly disclosed details of National Security Letters (NSL) received from the FBI, the first time a company has done so thanks to protections of the USA Freedom Act.

"We're able to disclose details of these NSLs today because, with the enactment of the USA Freedom Act, the FBI is now required to periodically assess whether an NSL's nondisclosure requirement is still appropriate, and to lift it when not," Chris Madsen, head of global law enforcement, security, and safety at Yahoo, wrote in a blog post.

NSLs allowed the FBI to demand data about a company's users without judicial review, and the companies that received them had to remain silent. There's been some back and forth over whether they are constitutional or not. But the passage of the USA Freedom Act last year allowed for more transparency.

As a result, Yahoo today disclosed NSLs it received in April 2013, August 2013, and June 2015.

"Specifically, we produced the name, address, and length of service for each of the accounts identified in two of the NSLs, and no information in response to the third NSL as the specified account did not exist in our system," Madsen wrote. "Each NSL included a nondisclosure provision that prevented Yahoo from previously notifying its users or the public of their existence."

The documents are redacted to protect the identities of the involved FBI agents, Yahoo personnel, and affected users.

"We believe this is an important step toward enriching a more open and transparent discussion about the legal authorities law enforcement can leverage to access user data," Madsen said. "Going forward, we will update our Transparency Report to include the number of NSLs received and number of accounts specified in those letters as the FBI lifts additional nondisclosure provisions."

In 2013, Google negotiated with the FBI so that it could reveal how many NSLs it receives, but only in very broad terms. For example, Google said it received between zero and 999 NSLs in 2012 regarding 1,000 to 1,999 users/accounts.