NASHVILLE, Tennessee (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.