Yahoo has called on the US government to explain why it compelled the company to scan millions of its users' email accounts. Yahoo's call, outlined in a letter published on Wednesday, comes after a report from Reuters showed that the web company built a powerful surveillance system that scanned all incoming emails for a specific string of characters. Yahoo built the system at the request of the US government in 2015, according to Reuters.

In a letter to James Clapper, the director of national intelligence, Yahoo's general counsel, Ron Bell, called on the government to clarify the "national security orders they issue to internet companies to obtain user data." The company said that although the letter references allegations made against Yahoo, "it is intended to set a stronger precedent of transparency for our users and all citizens who could be affected by government requests for user data."

"we find ourselves unable to respond in detail."

"We appreciate the need for confidentiality in certain aspects of investigations involving public safety or national security," the letter reads, "However, transparency is critical to ensure accountability and in this context must include disclosing how and under what set of circumstances the US government uses specific legal authorities, including the Foreign Intelligence Surveillance Act [FISA], to obtain private information about individuals' online activities or communications."

The American Civi Liberties Union (ACLU) also filed a motion this week before the Foreign Intelligence Surveillance Court, calling for the court to release the Yahoo order. As Reuters reports, the ACLU also asked the court to release more than 20 other orders that were previously issued.

Yahoo did not admit to building the email scanning system in the letter, nor did it detail the specific order it reportedly received to create it, saying "we find ourselves unable to respond in detail." National Security Letters and FISA court orders usually involve gag orders and are rarely made public. In a post published Wednesday, Yahoo characterized reports about its email scanning system as "misleading."

"As we've said before, recent press reports have been misleading; the mail scanning described in the article does not exist on our systems," the company said. "We therefore trust that the US government recognizes the importance of clarifying the record in this case," the company added.

Facebook, Google, Apple, and Microsoft said earlier this month that they have never received a surveillance request like the one Yahoo reportedly received.