Richard Farrell was fired in 2010 after the Postal Service learned he had burned letters instead of delivering them. He was convicted of mail theft in 2011 and sentenced to probation.
But authorities say they have now found much more mail on his property in Belfair, SeattlePI.com reported. Much of it was in postal tubs buried in a trench a friend excavated with a backhoe.
The total may amount to more than 200,000 pieces of mail, investigators said.
All the thefts took place before Farrell was charged in 2010. Authorities have not said if they plan a new prosecution.
The investigation was reopened after a woman postal worker who lived with Farrell reported the buried mail, officials said.
In the original case, investigators followed Farrell, who worked for the Postal Service from 1991 to 2010, on his route. They said he spent most of that time in a tavern and then took the mail home and burned it.
"His conduct reflects extreme laziness and a complete lack of consideration for the customers that he serviced," Assistant U.S. Attorney Thomas Woods said at the time. "Farrell repeatedly made the decision that he would rather spend his work hours not working, covering up his crime by stoking a fire pit with the very mail that he was supposed to be delivering."