MP Sergeant Jimmy Wycroft was a US soldier stationed at Area 51. When the base went on 24-hour lockdown due to nearby weapons testing, Wycroft, who hated the desert, was left in charge of two men, Corporal Higgins and Private Mitchell, at the front gate to keep out intruders. This included even officers of higher ranks, as Wycroft refused to allow a "Colonel Truman" to enter. However it turned out the "Truman" and all of his men were Soviets in disguise, and they killed Wycroft and his men.
Sgt. Wycroft is identified as simply MP Sergeant in the film credits of Indiana Jones and the Kingdom of the Crystal Skull.