Patrol Inspector William L. Sills was killed on January 17, 1940, while on night patrol duty at a crossing on the Rio Grande River in the vicinity of La Grulla, Texas. On the date mentioned, three Patrol Inspectors, William L. Sills, Albin Ulrickson, and Leslie H. Buchanan, encountered three smugglers in the act of smuggling contraband. Two of the smugglers were taken into custody, and Patrol Inspector Sills, in attempting to cut off the escape of the third smuggler, was shot at close range and fatally wounded. Before falling, Inspector Sills returned the smuggler's fire and killed him. The smuggler, a Mexican alien, had previously been deported from the United States.

Patrol Inspector Sills died in an ambulance enroute to the hospital in McAllen, Texas, where he was stationed. The two smugglers who were apprehended were arraigned on smuggling charges and for complicity in the killing.