Most states abandoned those execution methods more than a generation ago in a bid to make capital punishment more palatable to the public and to a judicial system worried about inflicting cruel and unusual punishments that violate the Constitution.
But to some elected officials, the drug shortages and recent legal challenges are beginning to make lethal injection seem too vulnerable to complications.
"This isn't an attempt to time-warp back into the 1850s or the wild, wild West or anything like that," said Missouri state Rep. Rick Brattin, who this month proposed making firing squads an option for executions. "It's just that I foresee a problem, and I'm trying to come up with a solution that will be the most humane yet most economical for our state."
Brattin, a Republican, said questions about the injection drugs are sure to end up in court, delaying executions and forcing states to examine alternatives. It's not fair, he said, for relatives of murder victims to wait years, even decades, to see justice served while lawmakers and judges debate execution methods.
Like Brattin, a Wyoming lawmaker this month offered a bill allowing the firing squad. Missouri's attorney general and a state lawmaker have raised the notion of rebuilding the state's gas chamber. And a Virginia lawmaker wants to make electrocution an option if lethal-injection drugs aren't available.
If adopted, those measures could return states to the more harrowing imagery of previous decades, when inmates were hanged, electrocuted or shot to death by marksmen.
States began moving to lethal injection in the 1980s in the belief that powerful sedatives and heart-stopping drugs would replace the violent spectacles with a more clinical affair while limiting, if not eliminating, an inmate's pain.
The total number of U.S. executions has declined in recent years — from a peak of 98 in 1999 to 39 last year. Some states have turned away from the death penalty entirely. Many have cases tied up in court. And those that carry on with executions find them increasingly difficult to conduct because of the scarcity of drugs and doubts about how well they work.
In recent years, European drug makers have stopped selling the lethal chemicals to prisons because they do not want their products used to kill.
At least two recent executions are also raising concerns about the drugs' effectiveness. Last week, Ohio inmate Dennis McGuire took 26 minutes to die by injection, gasping repeatedly as he lay on a gurney with his mouth opening and closing. And on Jan. 9, Oklahoma inmate Michael Lee Wilson's final words were, "I feel my whole body burning."
Missouri threw out its three-drug lethal injection procedure after it could no longer obtain the drugs. State officials altered the method in 2012 to use propofol, which was found in the system of pop star Michael Jackson after he died of an overdose in 2009.
The anti-death penalty European Union threatened to impose export limits on propofol if it were used in an execution, jeopardizing the supply of a common anesthetic needed by hospitals across the nation. In October, Gov. Jay Nixon stayed the execution of serial killer Joseph Paul Franklin and ordered the Missouri Department of Corrections to find a new drug.
Days later, the state announced it had switched to a form of pentobarbital made by a compounding pharmacy. Like other states, Missouri has refused to divulge where the drug comes from or who makes it.
Missouri has carried out two executions using pentobarbital — Franklin in November and Allen Nicklasson in December. Neither inmate showed outward signs of suffering, but the secrecy of the process resulted in a lawsuit and a legislative inquiry.
Michael Campbell, assistant professor of criminal justice at the University of Missouri-St. Louis, said some lawmakers simply don't believe convicted murderers deserve any mercy.
"Many of these politicians are trying to tap into a more populist theme that those who do terrible things deserve to have terrible things happen to them," Campbell said.
Richard Dieter, executive director of the Death Penalty Information Center in Washington, D.C., cautioned that there could be a backlash.
"These ideas would jeopardize the death penalty because, I think, the public reaction would be revulsion, at least from many quarters," Dieter said.
Some states already provide alternatives to lethal injection. Condemned prisoners may choose the electric chair in eight states: Alabama, Arkansas, Florida, Kentucky, Oklahoma, South Carolina, Tennessee and Virginia. An inmate named Robert Gleason Jr. was the most recent to die by electrocution, in Virginia in January 2013.
Arizona, Missouri and Wyoming allow for gas-chamber executions. Missouri no longer has a gas chamber, but Attorney General Chris Koster, a Democrat, and Missouri state Sen. Kurt Schaefer, a Republican, last year suggested possibility rebuilding one. So far, there is no bill to do so.
Delaware, New Hampshire and Washington state still allow inmates to choose hanging. The last hanging in the U.S. was Billy Bailey in Delaware in 1996. Two prisoners in Washington state have chosen to be hanged since the 1990s — Westley Allan Dodd in 1993 and Charles Rodman Campbell in 1994.
Firing squads typically consisting of five sharpshooters with rifles, one of which is loaded with a blank so the shooters do not know for sure who fired the fatal bullet. They have been used mostly for military executions.
Since the end of the Civil War, there have been three civilian firing squad executions in the U.S., all in Utah. Gary Gilmore uttered his famous final words, "Let's do it" on Jan. 18, 1977, before his execution, which ended what amounted to a 17-year national moratorium on the death penalty. Convicted killers John Albert Taylor in 1996 and Ronnie Lee Gardner in 2010 were also put to death by firing squad.
Utah is phasing out its use, but the firing squad remains an option there for inmates sentenced prior to May 3, 2004.
Oklahoma maintains the firing squad as an option, but only if lethal injection and electrocution are deemed unconstitutional.
In Wyoming, Republican state Sen. Bruce Burns said death by firing squad would be far less expensive than building a gas chamber. Wyoming has only one inmate on death row, 68-year-old convicted killer Dale Wayne Eaton. The state has not executed anyone in 22 years.
Jackson Miller, a Republican in the Virginia House of Delegates, is sponsoring a bill that would allow for electrocution if lethal injection drugs are not available.
Miller said he would prefer that the state have easy access to the drugs needed for lethal injections. "But I also believe that the process of the justice system needs to be fulfilled."
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