BUFFALO, Wyo. (AP) — Two people are dead after a heavily loaded semi-trailer lost its brakes on a steep slope on the eastern flanks of the Bighorn Mountains.

The Wyoming Highway Patrol identifies the victims as driver 60-year-old Robert D. Moffitt and passenger 58-year-old Margaret S. Moffitt, both of Leesburg, Fla.

Robert Moffitt was driving the semi-trailer loaded with sacks of bentonite early Tuesday evening when it lost its brakes coming down Powder River Pass.

The Highway Patrol report states that the semi-trailer was in too high a gear for the steep grade. The patrol states it's unknown why Moffitt didn't steer onto one of the runaway truck ramps on the slope.

The rig smashed through a guardrail and dropped 150 feet into Mosier Gulch, about 4 miles west of the town of Buffalo.