Portland, Ore.— Senator Ron Wyden (D-OR) is calling for a review of a government surveillance program that allows postal workers to monitor the mail.

A KGW investigation found over the past decade, more than 3,000 Oregonians have been the target of a little-known surveillance program called "mail covers."

“Given this high volume, it is important to make sure there are strong controls on the program to ensure that this technique is not used improperly,” wrote Wyden in a letter to Postmaster General Megan Brennan.

Read investigation: Secretive mail surveillance program targets Oregonians

Mail covers allow postal workers to record information from the outside of every letter that comes or goes from a particular address. The information is then turned over the law enforcement agency that requested it.

Unlike other government surveillance programs, like wiretaps, the use of mail covers does not require a judge’s approval.

Records obtained by KGW through the Freedom of Information Act showed the 3,114 requests for mail covers for Oregon addresses came from a variety of state and federal agencies including Alcohol, Tobacco and Firearms, Bureau of Diplomatic Security, Naval Criminal Investigative Service, U.S. State Department and U.S. Marshals Service. Local police and sheriff’s offices also called on postal workers to help snoop.

A 2014 federal audit found the United States Postal Service failed to follow key safeguards while running the mail covers program.

“I would like to request a formal update on whether these recommendations have been implemented or where in the implementation process the USPS is,” wrote Wyden.

Wyden's mail covers program letter