By Cole Hill ( | First Posted: Feb 20, 2013 01:17 PM EST

Nathan Dunlap, the "Chuck E. Cheese killer", just got an invitation to his last party ever. The U.S. Supreme Court rejected Dunlap's appeal Tuesday, effectively ensuring the first execution in Colorado in 15 years. 

Dunlap's rejection came as the the Supreme Court delivered a score of denials and approvals for upcoming appeals, the Christian Post reported. Following the appeal denial, his attorneys continued to argue their client's sentence should be commuted to life in prison without parole.

"Given what we know about the unfair and disproportionate use of capital punishment in Colorado, it would be unconscionable for the state to carry out this sentence," said Phil Cherner, one of Dunlap's attorneys, according to the Associated Press.

"Mr. Dunlap should spend the rest of his life in prison without the possibility of parole," he added. "What happened is tragic, but taking his life isn't going to change that."

Dunlap was sentenced to death in 1996 for the 1993 killing of four people at a Chuck E. Cheese restaurant in Aurora, Co. 

Prosecuting attorneys claimed that the then 19-year-old Dunlap consciously planned out the execution-style murders of four people. He was accused of fatally shooting three teenagers, and a 50-year-old woman, all in the back of the head, and robbing the business of $1,500 in cash and assorted "knickknacks." At the time, police said that Dunlap was seeking revenge on the restaurant chain, as he'd been fired from the exact location where his attack took place. 

Dunlap reportedly claimed he was mentally ill during the trial, as he thought the angle would have "mitigated his culpability enough to persuade at least one juror to vote for life imprisonment instead of death," according to the Post.

Dunlap's plan to claim insanity was foiled, though. The truth eventually came out in court that he'd casually explained to a doctor that, "I'm gonna play crazy as long as I can ... The police have no case against me, they're stupid," the Post reported.

The Supreme Court's rejection ends Dunlap's period of guaranteed appeals. His case will now be sent back to Colorado's 18th Judicial District to determine a timeframe for his execution, said Carolyn Tyler, spokeswoman for the Colorado Attorney General's office, Associated Press reported.

Dunlap's attorneys say they aren't done fighting for appeal in their client's execution. 

"I will continue to seek imposition of the death sentence in this case, in the interests of justice," said District Attorney George Brauchler to the Denver Post. "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."

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