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A Missouri death row inmate whose lethal injection was put on hold because he has a hole in his brain is arguing that the state could use nitrogen gas to end his life.

Lawyers for Ernest Lee Johnson, 56, got a scheduled execution halted in November by claiming that the execution drugs could trigger uncontrollable, painful seizures in violation of the constitutional protection against cruel and unusual punishment.

Death row inmate Ernest Lee Johnson is shown in this Missouri Department of Corrections photo released to Reuters on October 26, 2015. Missouri Dept of Corrections via Reuters

In new papers filed in federal court this week, his lawyers acknowledge that if they object to one method of execution, they are required by the U.S. Supreme Court to put forth an alternative — and they suggested nitrogen gas hypoxia, which starves the brain of oxygen.

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Missouri has two forms of execution on the book: lethal injection and lethal gas. It does not specify what kind of gas should be used and does not have a functioning gas chamber.

Nitrogen has never been used in an execution, but the defense cited a study commissioned by Oklahoma lawmakers on its feasibility.

A scan of Ernest John’s brain showing a hole in his skull and brain missing, from an affidavit of Dr. Joel Zivot.

"The available literature regarding the nitrogen gas method of execution strongly suggests that the subject will have no allergic reaction to the gas, will experience a loss of consciousness, and will suffer no pain," the attorneys wrote.

A spokeswoman for the Missouri attorney general's office had no immediate comment and said prosecutors would file their response in the next two weeks.

Related: Death Row Inmate Says Hole in His Brain Means He Shouldn't Be Executed

Any move to use untested nitrogen gas would likely delay Johnson's execution because of new legal challenges.

Johnson was sentenced to death for killing three convenience store workers with a claw hammer in 1994. In 2008, doctors removed part of a brain tumor, leaving a hole in the tissue. His lawyers say his "unique" defect means lethal injection is too risky.