Story highlights Tennessee death row inmates will be electrocuted if lethal injection drugs are unavailable

It is the only state to make electrocution mandatory

In other states that use the electric chair, it is an option

Drugs for lethal injections have been in short supply

As controversies over lethal injection drugs surge, Tennessee has found a way around the issue: It is bringing back the electric chair.

Eight states authorize electrocution as a method of execution but only at the inmate's discretion.

Now Tennessee is the first state to make use of the electric chair mandatory when lethal injection drugs are unavailable.

Tennessee Gov. Bill Haslam signed the measure into law Thursday.

"This is unusual and might be both cruel and unusual punishment," said Richard Dieter, president of the Death Penalty Information Center.

JUST WATCHED The death penalty in America Replay More Videos ... MUST WATCH The death penalty in America 01:22

JUST WATCHED Lethal Injection: The process Replay More Videos ... MUST WATCH Lethal Injection: The process 01:27

"No state says what Tennessee says. This is forcing the inmate to use electrocution," according to Dieter, who believes "the inmate would have an automatic Eighth Amendment challenge."

The amendment protects against cruel and unusual punishment.

"The electric chair is clearly a brutal alternative," Dieter said.

Controversy over lethal injections has been brewing in recent years after European manufacturers, including the Denmark-based manufacturer of pentobarbital, banned U.S. prisons from using their drugs in executions.

In April, a botched lethal injection in Oklahoma catapulted the issue back into the international spotlight. It was the state's first time using a new, three-drug cocktail for an execution. Execution witnesses said convicted murderer and rapist Clayton Lockett convulsed and writhed on the execution gurney and struggled to speak, before officials blocked the witnesses' view. Lockett died 43 minutes after being administered the first drug, CNN affiliate KFOR-TV in Oklahoma City reported.

Earlier this year, a convicted murderer and rapist in Ohio, Dennis McGuire, appeared to gasp and convulse for at least 10 minutes before dying from the drug cocktail used in his execution.

In 2009, the U.S.-based manufacturer of sodium thiopental, a drug also commonly used in executions, stopped making the painkiller.

Many states have scrambled to find products from overseas or have used American-based compounding pharmacies to create substitutes.

This month, a group of criminal justice experts recommended that federal and state governments move to a single lethal drug for executions instead of complex cocktails that can be botched.

The controversy over legal injection drugs raises the question of when a case will arise to test the new law.

The last death penalty by electrocution in Tennessee was that of Daryl Holton in 2007.

Holton -- a convicted murderer who killed his three young sons and his ex-wife's daughter -- elected to be killed by the electric chair.

Before Holton's execution, Tennessee had not used the electric chair in 47 years.