WASHINGTON, D.C. — Nathan Dunlap was sentenced to death for killing four people in an Aurora Chuck E. Cheese restaurant in 1993. Almost 20 years later, Colorado’s longest-serving death-row inmate saw his last, best chance to avoid execution thwarted by the U.S. Supreme Court.

That decision came down Tuesday morning, when a list of unsigned orders revealed that Dunlap’s case will not be heard by the country’s highest court.

This means an execution date can now be set for Dunlap. If it is carried out, it will make the 38-year-old the first inmate to be executed in Colorado since 1995.

In the appeal, Dunlap’s attorneys argued their client had been represented in his murder trial by incompetent lawyers who failed to introduce evidence that may have proved Dunlap’s mental instability.

Dunlap was just 19 years old when he walked into the Aurora restaurant where he had been fired and killed four of his former co-workers. The victims were restaurant manager Margaret Kohlberg, 50, Sylvia Crowell, 19, Ben Grant and Colleen O’Connor, both 17.