Twenty-one inmates on Missouri's death row have sued the state's Department of Corrections in federal court, alleging that the state's new lethal injection protocol -- which calls for a single injection of the powerful sedative propofol -- constitutes cruel and unusual punishment.
The Missouri Supreme Court has since halted all executions by lethal injection in the state as a result of the lawsuit, and now state Attorney General Chris Koster has hinted at bringing back the gas chamber.
The inmates claim in their lawsuit filed last August that Missouri cannot execute by lethal injection, because the injections now contain propofol, which has never been used before in any U.S. execution. The effects of propofol at lethal doses, they argue, are unknown and can never be tested in a clinical trial.
Koster said the federal litigation raises an "artificial hurdle … to prevent the state from carrying out the death penalty. Unless the court changes its current course, the legislature will soon be compelled to fund … alternative methods of execution to carry out lawful judgments," he said last week in a statement.
"Our state legalizes only two methods of execution, execution by lethal injection or execution by lethal gas," Nanci Gonder, a spokeswoman for the attorney general's office, told ABC News. "So if we couldn't execute by lethal injection, the alternative would be by lethal gas."
Lethal gas is "basically a painful process, no matter how you describe it," Richard Dieter, executive director of the Death Penalty Information Center, a nonprofit group that provides data on capital punishment, told ABC News.
"You drop pellets in an acid, which produces a gas that replaces the oxygen in the room. There are multiple ways of producing this gas. You can even use different gases. But either way you are slowly depriving the person of oxygen, making it difficult and then impossible for them to breathe."
Missouri last conducted an execution by lethal gas in 1968, according to Mandi Steele, a public information officer for the Missouri Department of Corrections. The state has since closed its lethal gas chambers, after lethal injections became a more common execution method.
"Currently, there are no operational facilities that are capable of conducting an execution by lethal gas," Steele said. "They would have to be rebuilt."
Dieter does not believe that Missouri will actually reinstate the gas chamber.
"This is just Koster's ploy to put pressure on the Missouri courts to expedite lethal injection executions," Dieter said. "Koster needs the courts to act before the state's supplies of propofol, the drug Missouri uses in lethal injections, runs out."
Missouri has only three batches of propofol left because the drug's manufacturer has stopped distributing it to correctional facilities in the United States. The oldest batch will expire in October 2013, and the newest batch will expire in 2015.
Koster explained the shortage in a request filed in the Missouri Supreme Court on July 1, 2013.
"As each supply expires, the department's ability to carry out lawfully imposed capital sentences diminishes," Koster said.
Missouri's lethal injection shortfall is the latest in a series of drug shortages that have limited states' ability to carry out executions by lethal injection.
"The companies that make the drugs that are normally used in lethal injections did not want their products to be used to carry out the death penalty. A lot of these companies are European and face strong pressure from the European Union to not export drugs into countries where they can be used for executions," Dieter said.
"So states had to scramble to find other drugs they could use. Missouri chose propofol, but now they're in the same situation they were in before," Dieter added.
On May 15, 2012, Missouri changed its lethal injection protocol to include a single lethal dosage of propofol; propofol was one of the drugs in the deadly mix that killed Michael Jackson.
On August 27, 2012, propofol manufacturer in the United States, Germany-based Fresenius Kabi, announced it would stop distributing its drug to American correctional facilities because the use of the drug for executions conflicted with the company's policy.
"We understand that one or more Departments of Correction in the U.S. are considering amend[ing] their lethal injection protocols to include propofol. Clearly, such use is contrary to the FDA approved indications for propofol and inconsistent with Fresenius Kabi's mission of 'caring for life," Scott Meacham, president of Fresenius Kabi's pharmaceutical division in North America, said in a letter to health care providers.
"To best prevent propofol from being used for purposes other than its approved indications, Fresenius Kabi does not accept orders for propofol from any departments of corrections in the U.S.," Meacham said in the letter.
Whether propofol can be used in lethal injections is at the center of Missouri's legal battle.
"That is why the inmates in Missouri are suing," Dieter said. "In order for the drug to be lethal, it needs to be administered in amounts that are much larger than normal. We don't know the side effects of the drug when it's taken in these amounts.
"Given that in the clinical setting, and at normal dosages, many patients experience significant pain on administration of propofol, it is highly likely that the incidence and severity of pain experienced by the prisoners subjected to the massive dose, specified by this protocol would be at least as high as are seen in smaller doses," the lawsuit states.
This pain, according to the lawsuit, constitutes cruel and unusual punishment.
The lawyers representing the death row inmates did not return several calls from ABC News seeking comment.
The Missouri Attorney General's Office countered the lawsuit with a motion to dismiss.
"Pain that is an 'inescapable consequence of death' does not meet the standard to fall under cruel and unusual punishment," Koster wrote.
Missouri is one in a number of states, including California, Maryland, Ohio and Texas, where inmates have sued to change lethal injection protocol.
"In California and Maryland, all executions were halted until the legal proceedings had concluded," Dieter told ABC News. "In Ohio and Texas, the executions were carried on."
For Koster, the inmates' lawsuit against Missouri is a way of stalling the inevitable.
The lawsuit, he said in his recent statement, "has been used by convicted killers as a delay tactic to prevent the state from carrying out lawful judgments."