The box of 12 assault rifles was "probably off the yard fairly quickly," said Maj. Shannon Smith.

And that means the small caliber rifles, which Smith estimated are worth up to $5,000 each on the street, now could be anywhere.

Hill Air Force Base investigators do have a few leads. The box appears to have fallen off a truck belonging to an as-of-yet unidentified unit at Hill at about 3:45 p.m. Several witnesses said they saw the unidentified green box in the road and several claimed to have seen a middle-age white male, medium height and heavy-set, with gray hair and wearing civilian clothes, lifting it into a car.

The base will be working with a forensic sketch artist to build a composite. But Smith acknowledged that in a base with more than 20,000 civilian workers, the general description could fit a lot of people.

Hill Air Force Base commander Kathleen Close and Air Base Wing Commander Linda Medler refused through a spokesman to answer questions about the unit involved.

Hill has been embroiled in a chain of mishaps and scandals recently, including a friendly fire incident at the Utah Test and Training Range, the accidental burning of radioactive materials at a civilian incinerator and the mistaken transfer of nuclear missile parts to Taiwan. The latter debacle was in part responsible for Thursday's resignation of the Air Force's two top officials.

In the case of the missing machine guns, Smith said the unit that lost the weapons was returning from field exercises. It had loaded the container onto the flatbed of a Humvee, but failed to secure the case, which apparently fell out of the truck just south of the base's Roy Gate.

By the time the unit's airmen arrived at their destination and realized the weapons were missing, several commuters had already reported to base security that they had spotted an nondescript green box in the road. But before base security personnel returned to the place where the weapons fell, the box was gone.

Smith said it might have taken up to an hour before all base gates were shut down and checks were being made of all cars leaving the base. But he also said that the person who picked up the weapons easily could have had them off the base before security had even been alerted as to the contents of the green box.

mlaplante@sltrib.com