A deputy who was a member of the office's SWAT Team took the rifle before being deployed to Afghanistan in 2006. He left the rifle in his gun safe at home before leaving for military duty, according to a Thursday press release from the sheriff's office.
The weapon was one of 20 the U.S. Department of Defense gave to the sheriff's office in 1998.
An audit conducted in 2013 revealed the rifle was missing and all Utah law enforcement agencies in the state were suspended for a month from the DOD program, that gives surplus weapons to law enforcement agencies.
The audit concluded the rifle went missing before 2006.
An investigation by the sheriff's office showed that for the most part, the weapons remained in storage until 2006 when they were issued to deputies.
Thursday's press release said the deputy requested permission to take an M-16 to train for SWAT before he was deployed.
After the training, the deputy placed the gun into his gun safe at home, where it remained until last week. The release said the deputy completed his tour, returned home and did a second tour of duty in the Middle East.
He has been home for several years.
"During the investigation, partially because there was a lack of paperwork, and partially because of human error, the employee never heard about an investigation into a missing M-16 rifle," the press release said. "It wasn't until he was reading in the newspapers (last week) that DCSO had a missing assault rifle that his memory was sparked"
He then checked his gun safe and called his superior. Davis County Sheriff Todd Richardson was notified Friday, April 11 and the investigation was reopened.
The gun was never on the streets being used for criminal purposes, according to the press release.
"He (the deputy) simply forgot it was there," the press release said.
The deputy faces administrative action and the report has been forwarded to the Davis County Attorney's Office for review.