Warren Lee Hill, whose defenders say he is mentally disabled, is scheduled to die by lethal injection Tuesday for the 1990 killing of Joseph Handspike. In an undated photograph Warren Hill poses with his sister Peggy Williams and his nephew. (23850)

FULTON, Ga. — A court in Fulton County, Georgia, has temporarily stopped the scheduled Monday night execution of condemned murderer Warren Hill.

Hill was sentenced to death for the 1990 killing of Joseph Handspike, another inmate in a Georgia state prison.

He was convicted of beating Handspike to death with a nail-studded board while serving a life sentence in the 1985 killing of his girlfriend, Myra Wright. His lawyers have argued that Hill is mentally retarded.

“Today, the Court found that more time is needed to explore Mr. Hill’s complaint, which raises serious concerns about the extreme secrecy surrounding the execution process in Georgia, and the new Lethal Injection Secrecy Act, which took effect one day before Georgia issued a death warrant for Mr. Hill,” Brian Kammer, Hill’s attorney said in a statement.

Monday is not the first time Hill’s execution has been halted.

He had previously been scheduled to die last July, but the state Supreme Court stopped the execution on procedural grounds. Hill was granted another stay in February.

According to Kammer, a briefing on Hill’s complaint will take place Thursday. A new execution date is expected to be set for the same day, he said.

“Ultimately, we are hopeful that the United States Supreme Court will hear Mr. Hill’s pending Petition for Writ of Habeas Corpus, and will have the opportunity to consider the important new evidence in this case, that there is unanimous consensus among all the doctors who have examined Mr. Hill, including three who previously testified for the state, that he is a person with mental retardation, and thus ineligible for the death penalty,” said Kammer.

Hill’s lawyers say his low IQ means he should be spared under a 2002 decision that barred the execution of the mentally disabled.

A string of state courts has said Hill doesn’t qualify under Georgia law, which requires inmates to prove mental impairment “beyond a reasonable doubt.”

CNN’s Matt Smith and Dana Ford contributed to this report.