"Right now, it's administrative," Poulsen said of the investigation. "We're not sure [whether a crime has been committed] until it gets a little bit closer" to completion.
A probe last year pointed to a current or former peace officer taking the rifle.
The Defense Department provided 20 M-16s to the Davis County Sheriff's Office through the Pentagon's 1033 Program, which gives surplus weapons and other gear to law enforcement.
When a March 2013 audit revealed one M-16 in Davis County was missing, all of Utah was suspended from the 1033 Program, according documents the state provided to The Salt Lake Tribune. Utah was reinstated the next month, when Davis County adopted a corrective-action plan.
The sheriff's office investigation found that except for a few training exercises, the rifles sat in storage until 2006, when they were issued to deputies. But there's no record of the missing rifle having been issued, according to a report from the sheriff's own detectives. Investigators also couldn't find the mechanism disabling the missing rifle's automatic fire capability. The mechanisms were removed from the other M-16s in 2006.