The U.S. Supreme Court lifted a second stay late on Thursday within hours of the death sentence for a former Eagle Scout condemned in a 1994 convenience store killing, clearing the way for his execution.

Ronald Smith, 45, had originally been set to die by lethal injection at 6 p.m. CST (midnight GMT) at the Holman Correctional Facility in Atmore, Alabama, before the Supreme Court twice ordered his execution put on hold with just hours to spare.

The court later lifted the second hold at about 9 p.m. CST (0300 GMT), after which his attorneys immediately filed a petition for another stay.

The Supreme Court offered no explanation for any of the three orders it issued in the case on Thursday.

If Smith is put to death, he would be the 20th person executed in the United States this year and the second in Alabama, according to the non-profit Death Penalty Information Center.

The Supreme Court granted a last-minute hold to stop Alabama carrying out another man's death sentence a month ago. Justices also did not give a reason for the stay of execution in that case.

Alabama's death penalty process is under scrutiny after the high court ruled in January that a similar death penalty law in Florida gave too much discretion to judges.

Smith was convicted of murdering Casey Wilson, a convenience store clerk in Huntsville, during a failed robbery.

The jury that convicted Smith recommended a sentence of life in prison without parole. However, trial judge Lynwood Smith, now a federal judge, imposed a death sentence, as allowed by state law.

According to trial testimony, Smith had been an Eagle Scout and a member of the National Honor Society but struggled with alcoholism as an adult.

In an appeal to the U.S. Supreme Court, Smith's attorneys argued that his death sentence should be overturned.

They contended that Alabama's process was similar to Florida's, which the court struck down this year. The justices ruled that Florida judges were given powers that juries should wield in deciding eligibility for the death penalty.

The U.S. Supreme Court has since ordered Alabama to review similar practices in four other cases not involving Smith, according to court documents. Those reviews are pending.

Alabama argues that its law is different from Florida's and that the Supreme Court ruling in the Florida case was not retroactive to earlier cases.

(Reporting by David Beasley in Atlanta; Additional reporting by Dan Whitcomb in Los Angeles; Editing by Jonathan Oatis, Leslie Adler and Paul Tait)