A Missouri inmate’s execution was put on hold at the last minute by the U.S. Supreme Court, sending the case back to an appeals court for further review. Death row inmate Ernest Lee Johnson was scheduled to be executed Tuesday at the Missouri state prison in Bonne Terre.

The 55-year-old claimed in his appeal that the lethal drug used in the execution could cause painful seizures as he has a slow-growing tumor in his brain. Defense attorney Jeremy Weis reportedly said that a 2008 surgery to remove Johnson's tumor forced removal of up to 20 percent of his brain tissue, and that he has had prior seizures.

Johnson was convicted of three counts of first-degree murder for beating three people to death with a claw hammer during a robbery of a Casey's General Store in Columbia on Feb. 12, 1994.

The Supreme Court granted a stay on Johnson’s execution while the 8th U.S. Circuit Court of Appeals considers whether his complaint was properly dismissed, the Associated Press (AP) reported. However, it is unclear when the appeals court will issue a ruling in the case.

"There is no indication of any kind that this is going to be resolved tomorrow," Mike O'Connell, a spokesman for the Missouri Department of Corrections, told AP Tuesday.

Missouri reportedly has 24 hours to fulfill the execution -- in this case until Wednesday evening -- or authorities will have to reschedule the execution.

In a second appeal, Johnson’s lawyer claimed that his client is mentally disabled and his life should be spared.

Prosecutors have reportedly said in court documents that both claims are without merit.

Twenty-five people have been so far executed in the United States this year, according to the Death Penalty Information Center, including six in Missouri.