The U.S. Supreme Court denied Nathan Dunlap a death penalty appeal on Tuesday, sentencing him to death for killing four people in an Aurora Chuck E. Cheese restaurant in 1993, according to PIX 11.

Almost 20 years after the crime was committed, an execution date can now be set for the 38-year-old, who is also Colorado's longest-serving death-row inmate. It the execution is carried out, Dunlap will be the first inmate executed in Colorado since 1995, according to the report.

At the age of 19, Dunlap walked into a Chuck E. Cheese restaurant where he had been fired and killed four of his former co-workers. He reportedly hid in the bathroom at the restaurant until closing and then shot three teenagers and the store manager who was a mother of two, according to Yahoo. He was convicted in 1996, three years after the crime, of four counts of capital murder as well as other crimes. He was sentenced to death on the murder counts and terms that consecutively total 113 years on the other counts. The sentence was followed by Colorado Supreme Court.

In the court appeal, his attorneys argued that Dunlap was represented by inadequate lawyers during his murder trial who failed to bring forth evidence that would prove their client's mental instability.

An attorney for Dunlap argued that his client should serve life in prison without parole and that the death penalty in Colorado would be "unfair and disproportionate," according to the Denver Post. Dunlap's motion for sentence reconsideration and his motion for post-conviction relief were also denied.