OKLAHOMA CITY - A federal judge plans to rule whether the lethal injection formula that Oklahoma used for the first time in a controversial execution last spring is constitutional.

U.S. District Judge Stephen Friot said he will rule from the bench later Monday in a case in which 21 death row inmates say the state's use of untested execution drugs and drug combinations on them would amount to experimentation and is unconstitutional.

They 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.

That was the first time Oklahoma used the sedative midazolam, which the inmates say isn't appropriate for executions.

The state says its new execution protocols are constitutional.

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