MONTGOMERY, Ala. (AP) — A federal appellate court upheld a stay of execution Wednesday for an Alabama inmate who is challenging the humaneness of the state's lethal injection procedure.

Torrey Twane McNabb, 40, was scheduled to be put to death by lethal injection on Thursday for the 1997 shooting death of a police officer in Montgomery, Ala.

A three-judge panel of the 11th U.S. Circuit Court of Appeals upheld a stay issued by a lower court judge to allow for more proceedings in McNabb's lawsuit, which contends that Alabama plans to use an unreliable sedative at the start of the procedure.

The Alabama attorney general's office said it will ask the U.S. Supreme Court to let the execution proceed, arguing that justices have already allowed four lethal injections to be carried out in Alabama using the sedative.

McNabb was convicted of killing Montgomery police officer Anderson Gordon in 1997. Prosecutors say McNabb shot Gordon multiple times after he arrived at a traffic accident that McNabb caused while fleeing a bail bondsmen.

McNabb is one of several inmates in an ongoing lawsuit challenging the state's use of the sedative midazolam at the start of executions, saying it would not reliably render them unconscious before other drugs stopped their lungs and heart. The 11th Circuit last month said a federal judge prematurely dismissed their lawsuit, and ordered the judge to hold additional proceedings.

"This court ordered that Mr. McNabb was entitled to further proceedings on the merits of his case challenging the constitutionality of Alabama's execution protocol," John Palombi, an attorney for McNabb, wrote in the filing to the 11th Circuit.

The state has argued McNabb has presented no new information to merit blocking the execution.

"Alabama has already carried out four executions using this protocol. Three of those executed inmates were co-plaintiffs in this case, and their stay requests were denied by both the Supreme Court and this Court," the Alabama attorney general's office told the 11th Circuit.