OKLAHOMA CITY (AP) — Oklahoma's lethal injection protocol is 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 Oklahoma death row inmates. The prisoners 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.

After the ruling, Department of Corrections Director Robert Patton said the state planned to move forward with the execution of Charles Frederick Warner on Jan. 15 and three other lethal injections scheduled through March 5.

"We will now proceed with the guidelines set forth in the policy and protocol in preparation for the upcoming executions," Patton said.

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.

In Arizona, where a combination of midazolam and the painkiller hydromorphone was used during a nearly two-hour execution in July, state officials said Monday they planned to switch to a three-drug combination similar to Oklahoma's.

Prison officials in both Oklahoma and Arizona have said they would prefer to use the barbiturates pentobarbital or sodium thiopental, but both drugs have become difficult for states to obtain for executions.

Attorneys for Oklahoma maintained the problems with Lockett's execution were the result of an improperly set single IV line that wasn't properly monitored during his execution, causing the lethal drugs to be administered locally instead of directly into his blood. The protocol the state adopted after Lockett's execution calls 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.

Patton, who testified during a three-day hearing last week, said he believes Florida's protocol is "humane."

Judge Friot said in his ruling from the bench that he placed "considerable reliance" on the ability of the execution team to have a backup IV line, to constantly monitor the IV lines, and to ensure that an inmate is unconscious before the second and third drugs are administered.

In addition to adopting a new execution protocol, Oklahoma has bought new medical equipment and ordered more training for the execution team. Prison officials say they're prepared for the upcoming executions.

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

Dale Baich, one of the attorneys for the Oklahoma death row inmates, said in a statement they plan to appeal Friot's decision.

"As anesthesiologists and other medical experts have detailed, our primary concern is the use of midazolam, a drug that is inappropriate for use in executions because it does not relieve pain and does not maintain prisoners at an adequate level of anesthesia," Baich said. "And because Oklahoma plans to paralyze condemned prisoners after giving them midazolam, it is likely we often will not know if the prisoners were medically and constitutionally anesthetized or if they suffered."

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Associated Press writer Astrid Galvan contributed to this report from Tucson, Arizona.

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Sean Murphy can be reached at www.twitter.com/apseanmurphy .