A New York Times investigation has revealed the U.S. Postal Service approved thousands of requests from federal agencies to monitor the public's mail as part of its extensive surveillance program.

In 2013, the Postal Service approved some 50,000 requests from federal authorities and its own Postal Inspection Service investigative branch to keep taps on information written on mail packaging, including names and addresses, before delivery. In some cases a computer takes a picture of paper mail which is later used by investigators to view images of communication between suspects, according The NY Times. The mail is not opened.

The Post Office's surveillance program, titled mail covers, dates back more than a century and is used to help federal law enforcement conduct criminal investigations.

But a 2014 audit of mail covers obtained by The NY Times brought up concerns of abuse of citizens' rights with the way the Postal Service conducts the surveillance program. Many requests were approved without written consent or a proper reason. Others weren't processed on time and computer glitches led to multiple requests being listed under the same tracking number, according to the audit.

"Insufficient controls could hinder the Postal Inspection Service's ability to conduct effective investigations, lead to public concerns over privacy of mail and harm the Postal Service's brand," reads the audit, which Politico first reported about in June.

One surveillance request involved a defense attorney from San Antonio who found out federal prosecutors were monitoring mail sent between her and her client in connection to a pornography case about a decade ago. The practice is banned under Postal Service policy, The NY Times reported.

The lawyer, Cynthia Orr, expressed her grievances but still doesn't know if the prosecutors stopped the surveillance.

"The troubling part is that they don't have to report the use of this tool to anyone," Orr told the newspaper.

The Postal Service did not comment on the issue. But the agency did say it has policies in place to protect the rights of Americans and it is not as invasive as the National Security Agency.

"You can't just get a mail cover to go on a fishing expedition," Postal Inspection Service spokesman Paul Krenn told the newspaper. "There has to be a legitimate law enforcement reason, and the mail cover can't be the sole tool."