Oklahoma's lethal injection protocols are constitutional and the state can proceed with the scheduled executions of four death row inmates early next year, a federal judge ruled on Monday.

U.S. District Judge Stephen Friot denied a request for a preliminary injunction that was requested by a group of 21 Oklahoma death row inmates who argued the use of the sedative midazolam as the first drug in a three-drug combination the state administers risks subjecting them to unconstitutionally cruel and unusual punishment.

The inmates sued after the April 29 execution of Clayton Lockett, who writhed on the gurney, mumbled and lifted his head during his 43-minute execution that the state tried to halt before it was over. Lockett's execution was the first in Oklahoma using midazolam, which also has been used in problematic executions in Ohio and Arizona.

Attorneys for the state maintained the problems with Lockett's execution were the result of an improperly set single intravenous line that wasn't properly monitored during his execution, causing the lethal drugs to be administered locally instead of directly into his blood. The protocols the state adopted after Lockett's execution call for a five-fold increase in the amount of midazolam used, which is the same amount of the drug used in 11 successful Florida executions.

The director of the state's prison system, Robert Patton, was one of several witnesses who testified during a three-day hearing last week and said he believes Florida's protocol is "humane."

But medical experts called by attorneys for the death row inmates testified that midazolam won't properly anesthetize a person and render them unconscious for the second drug, which causes them to suffocate, and a third which would cause a burning pain before stopping the heart.

The state has purchased new medical equipment, adopted new execution protocols and ordered more training, and prison officials say they're ready for the execution of Charles Frederick Warner on Jan. 15. Three other lethal injections have been scheduled through March 5.

The Associated Press