After two decades on death row, Nathan Dunlap, known as the Chuck E. Cheese killer who gunned down four employees after being fired from the Aurora location, could now face execution in Colorado.

According to the Denver Post, the U.S. Supreme Court rejected the 38-year-old’s death penalty appeal on Tuesday. Dunlap was convicted in 1996 for killing four people in 1993 at an Aurora, Colo., Chuck E. Cheese from which he was fired.

The high court said it would not hear the last appeal for Dunlap guaranteed under law, which clears the way for an execution date to be set for Colorado’s longest-serving death row inmate.

"We're one step closer," former Arapahoe County District Attorney and prosecutor in the case Jim Peters told the Denver Post. "It's not finalized. But we've taken another major step forward."

The current DA, George Brauchler, told the Denver Post, he will “continue to seek imposition of the death sentence in this case.”

"Our office has spent 19 years prosecuting Nathan Dunlap for the preplanned and deliberate murders of the unsuspecting three teenagers and one adult victim who had the terrible misfortune to be working the night shift on Dec. 14, 1993, at Chuck E. Cheese in Aurora," Brauchler said.

Dunlap could file further appeals but it is unlikely those appeals could postpone his death sentence in Colorado, a state which has not executed an inmate since 1997.

In 1993, Dunlap open fired after the Chuck E. Cheese restaurant closed, killing victims Sylvia Crowell, 19, Benjamin Grant, 17, Margaret Kohlberg, 50, and 17-year-old Colleen O’Connor. Another employee, Bobby Stephens, survived the shooting after being shot in the face and helped to identify Dunlap. According to the case, the shooting happened after Dunlap was fired from his job at the restaurant and wanted to “get even.”

Dunlap was arrested the following day, on Dec. 15, 1993, and pled guilty the following year in exchange for life in prison. He was sentenced to death in 1996 after being charged with eight counts of first-degree murder.

Since 1996, Dunlap’s death sentence has been upheld as Dunlap and his lawyers battled in court for a life in prison sentence, claiming his legal team was incompetent. Dunlap also argued that he was mentally ill, but according to the court, Dunlap said to a doctor, "I'm gonna play crazy as long as I can. ... The police have no case against me, they're stupid” and added he would kill again.

With Tuesday’s decline for his appeal, Dunlap has exhausted the amount he was legally guaranteed, though one of his attorneys, Phil Cherner, is working on a petition for clemency.

"Mr. Dunlap should spend the rest of his life in prison without the possibility of parole," Cherner told the Denver Post. "What happened is tragic. But taking his life isn't going to change that."