The Postal Service now thinks it knows.
Authorities say they have recovered about 20,000 pieces of mail from the garage of a Philadelphia postal carrier.
The carrier worked in the city's Bustleton neighborhood - but apparently didn't work as much as he was supposed to.
Some of the mail that was found dates back to 1997.
It took three trucks to remove the mail - and authorities have been delivering the missing letters and parcels to customers since last week.
Postal officials haven't identified the carrier - and are still trying to find him so they can ask him what happened.
