A glance at the five execution methods allowed today
FILE - This July 25, 2014 file photo shows bottles of the sedative midazolam at a hospital pharmacy in Oklahoma City. Exactly one year after a botched lethal injection, attorneys for other Oklahoma death row inmates were set to ask the U.S. Supreme Court Wednesday, April 29, 2015 to outlaw a sedative used in the procedure â a ruling that could force several states to either find new execution drugs or change the way they put prisoners to death. (AP Photo/Sue Ogrocki, File)
In this Oct. 9, 2014 file photo, Department of Corrections officials are pictured in the witness room at right, outside the newly renovated death chamber at the Oklahoma State Penitentiary in McAlester, Okla. With executions in Oklahoma on hold amid a constitutional review of its lethal injection formula, Republican legislators are pushing to make Oklahoma the first state in the nation to allow the use of nitrogen gas to execute death row inmates. (AP Photo/Sue Ogrocki, File)
An unidentified Arizona Corrections Officer adjusts the straps on the gurney used for lethal injections at the Florence Death House at the Arizona State Prison at Florence (Ariz.) in this undated photo provided by the Arizona Department of Corrections. On Tuesday, Sept. 2, 2003, a federal appeals court overturned more than 80 death sentences in Arizona because a judge, instead of a jury, handed them down. Death sentences in Idaho and Montana also were affected. (AP Photo/Arizona Department of Corrections)
File - In this April 12, 2005 file photo is the death chamber at the Missouri Correctional Center in Bonne Terre, Mo. The Associated Press and four other news organizations filed a lawsuit Thursday, May 15, 2014 challenging the secret way in which Missouri obtains the drugs it uses in lethal injections, arguing the state's actions prohibit public oversight of the death penalty. The suit asks the state's department of corrections to disclose where it purchases drugs used to carry out executions along with details about the composition and quality of those drugs. (AP Photo/James A. Finley, File)
FILE - In this Nov. 2005 file photo, public information director Larry Greene is shown in the death chamber at the Southern Ohio Correctional Facility in Lucasville, Ohio. Ohio prison officials said Friday, Oct. 4, 2013, they are keeping their primary lethal injection drug in place despite the state's supply expiring, but they've added a second drug option for executioners to address the shortage. Prisons spokeswoman JoEllen Smith said the powerful sedative pentobarbital will remain Ohio's primary method of administering the death penalty. A policy posted to the prisons department's website listed a combination of midazolam and hydromorphone as an alternative if sufficient pentobarbital isn't available or if the existing supply "is deemed unusable" by the medical team. (AP Photo/Kiichiro Sato, File)
393846 06: A gurney and a electric chair sit in the death chamber of the Southern Ohio Correctional Facility August 29, 2001 in Lucasville, Ohio. The state of Ohio is one of the few states that still uses the electric chair, and it gives death row inmates a choice between death by the electric chair or by lethal injection. John W. Byrd, who will be executed on September 12, 2001, has stated that he will choose the electric chair. (Photo by Mike Simons/Getty Images)
393846 05: A view of the death chamber from the witness room at the Southern Ohio Correctional Facility shows an electric chair and gurney August 29, 2001 in Lucasville, Ohio. The state of Ohio is one of the few states that still uses the electric chair, and it gives death row inmates a choice between death by the electric chair or by lethal injection. John W. Byrd, who will be executed on September 12, 2001, has stated that he will choose the electric chair. (Photo by Mike Simons/Getty Images)
FILE - This is an undated file photo of the electric chair at the Tennessee State prison in Nashville. First used by New York State in 1890, it was used throughout the 20th century to execute hundreds and is still an option in eight states. Since 1976, 158 inmates have been executed by electrocution. It was considered humane on its introduction but resulted in many horrific executions over the years. (AP Photo, File)
This is a 1996 photo of Yellow Mama, Alabama's electric chair at Holman Prison in Atmore, Ala. Alabama Gov. Don Siegelman and Attorney General Bill Pryor want lethal injections legalized in Alabama, but only as a precaution in case the courts declare the electric chair unconstitutional. (AP Photo)
This undated photo provided by the Virginia Department of Corrections shows an electric chair. Larry Bill Elliott is scheduled to be executed by choice of electrocution Tuesday, Nov. 17, 2009 for the January 2001 shooting deaths of 25-year-old Dana Thrall and 30-year-old Robert Finch. (AP Photo/Virginia Department of Corrections)
This photo taken May 16, 2013, shows an electric chair on exhibit at the Texas Prison Museum in Huntsville, Texas. Between 1924 and 1964, 361 men died in the electric chair. Since the first execution by lethal injection in Texas in 1982 the state has executed 499 prisoners. (AP Photo/Pat Sullivan)
This is the interior of the gas chamber at the Nevada State Penitentiary in Carson City, Nev., seen 1936. (AP Photo)
FILE - In this June 18, 2010, file photo, the firing squad execution chamber at the Utah State Prison in Draper, Utah, is shown. In the wake of a bungled execution in Oklahoma last month, a Utah lawmaker wants to resurrect firing squads as a method of execution in his state. Rep. Paul Ray, a Republican from Clearfield, says firing squads would be a quick and humane way to put someone to death as lawsuits and drug shortages have hampered lethal injections in recent years. Ray plans to introduce his proposal during Utahâs next legislative session in January. Utah stopped allowing death-row inmates to choose execution by firing squad after 2004. Several inmates sentenced before that time have opted for firing squad executions but are appealing their sentences. Utah last used the method in 2010, when a firing squad of five police officers with .30-caliber Winchester rifles executed Ronnie Lee Gardner. (AP Photo/Trent Nelson, Pool, File)
The execution chamber at the Utah State Prison after Ronnie Lee Gardner was executed by firing squad Friday, June 18, 2010. Four bullet holes are visible in the wood panel behind the chair. Gardner was convicted of aggravated murder, a capital felony, in 1985. (AP Photo/Trent Nelson - Pool)
The execution chamber at the Utah State Prison is seen after Ronnie Lee Gardner was executed by firing squad Friday, June 18, 2010 in Draper Utah. Four bullet holes are visible in the wood panel behind the chair. Gardner was convicted of aggravated murder, a capital felony, in 1985.(AP Photo/Trent Nelson/Pool)
The execution chamber at the Utah State Prison after Ronnie Lee Gardner was executed by firing squad Friday, June 18, 2010. The bullet holes are visible in the wood panel behind the chair. Gardner was convicted of aggravated murder, a capital felony, in 1985. (AP Photo/Trent Nelson - Pool)
A huge crowd of over 15, 000 people gathers around a scaffold to witness the public hanging of 22-year old Rainey Bethea August 14, 1936 in Owensboro, Kentucky. Public outrage over the manner of execution made Bethea's death the last public hanging in the country. (Photo by Newsmakers)
NASHVILLE, Tenn. (AP) - Five execution methods are legal in various places in the United States: injection, electrocution, gas, firing squad and hanging. Tennessee this week became the first state to allow use of the electric chair in some circumstances regardless of the inmate's wishes, if injection drugs are not available.
However, all 35 states that have death row inmates, as well as the U.S. military and the federal government, use injection as their primary method of execution, according to the Death Penalty Information Center.
Deborah Denno, a professor at Fordham Law School who has studied executions for more than two decades, said states have changed execution methods over the years in recurring attempts to make them more humane and to avoid litigation.
Here is a look at how each of those methods specifically causes death:
First adopted in 1977 in Oklahoma, lethal injection has become the method of choice in all states that still carry out executions.
Generally, inmates are strapped to a gurney while needles are inserted into the veins and the drugs are pumped in. This method is often seen as the most humane of the five because the inmates are supposed to be sedated before they die. Inmates, though, have been known to writhe and talk during poorly carried out injections.
According to Denno, until 2009, all states used a three-drug protocol that included a sedative, a paralytic and then the final, fatal drug to stop the heart. Because of drug shortages and legal challenges that claimed the paralytic drug could mask an inmate's suffering, states are now experimenting with several different protocols.
Some states are adopting a one-drug method that is essentially a massive overdose of a sedative. Other states are keeping a multi-drug protocol but experimenting with different drugs.
New York developed electrocution as an alternative to hanging - which was often a gruesome public spectacle - and executed the first inmate by electric chair in 1890.
Prisoners generally are strapped into a chair with electrodes placed on their heads and legs. Saline-soaked sponges are placed between the skin and the electrodes to aid conductivity.
Denno said the voltage, the number of jolts and the length of time they are administered vary from state to state. Executioners usually give more than one jolt of electricity, to make sure the inmate is dead. Executioners can't give one long, continuous jolt because the person's body could start to burn. Instead they let the body cool down for a few seconds between jolts.
It is unknown whether the person being electrocuted is rendered unconscious by the shock or is merely paralyzed and unable to yell out.
Denno said electrocution usually kills by sending the inmate into cardiac arrest, but it could also cause brain death first. "Or it could be both brain death and heart death."
After Tennessee executed Daryl Holton by electric chair in 2007, a method he chose, state medical examiner Dr. Bruce Levy said Holton died when the electricity stopped his heart. Holton also had burns where the electrodes contacted the skin. And Levy said inmates sometimes suffer broken bones when their muscles clench violently during the shock, but that did not happen with Holton.
Nevada developed the gas chamber in the 1920s as an attempt at a humane method of execution, but Denno said it had "horrific problems" from the start. The original idea was to pump the gas into an inmate's cell while he was sleeping, but there was no way to keep the gas contained, so they built a chamber instead.
Inmates are strapped into a chair and the chamber is filled with cyanide gas, which kills by asphyxiation. The inmates are fully awake and conscious as they suffocate, Denno said.
This method has been used as recently as 2010 in Utah at the request of a condemned man there.
Denno said the prisoner is strapped to a chair, as in electrocution and the gas chamber. A cloth target is placed over prisoner's the heart. Several shooters are given real bullets but one or more are given blanks. Assuming the shooters hit their target, the heart ruptures and the prisoner dies quickly from blood loss.
Before 1890, hanging was the principal method of execution across the country. The prisoner stands over a trap door while a noose is placed around the person's neck, and then the trap door is opened and the prisoner falls.
By design, the fall breaks the prisoner's neck and kills him or her, but Denno said that has often not been the case. In some cases, prisoners have been decapitated from the fall. In other cases, they have strangled over the course of several minutes.