The U.S. Supreme Court on Monday declined to consider Warren Hill’s bid to halt his execution on claims he is mentally retarded.

Hill’s lawyers had filed the petition directly to the high court, saying they now have evidence that shows they can prove Hill is mentally retarded and thus ineligible for execution. But the high court on Monday issued an order denying it.

In a statement, Brian Kammer, one of Hill’s lawyers, said he was “gravely disappointed” the court declined to hear the case.

“It is the unanimous opinion of all doctors who have examined him that Mr. Hill is a person with mental retardation,” Kammer said. “However, Mr. Hill has been procedurally barred from proving his exemption from capital punishment, which is why he brought his case to the U.S. Supreme Court, in the hopes that the court would ensure that the evidence of his intellectual disability would be heard. It is tragic that our highest court has failed to enforce its own command that persons with mental retardation are categorically ineligible for the death penalty.”

Hill’s execution remains on hold while the Georgia Supreme Court hears his challenge to a new state law that keeps secret the identities of those who make and supply Georgia’s lethal-injection drugs.

Hill, 53, sits on death row for beating another inmate, Joseph Handspike, to death with a nail-studded board in 1990 at the state prison in Leesburg. At the time, Hill was serving a life sentence for the 1986 fatal shooting of his former girlfriend.