Updated

At 8:23 p.m. local time on Sept. 25, 2007, Michael Richard was pronounced dead inside the execution chamber at the Texas State Penitentiary in Huntsville. The United States hasn't held an execution since.

Just a few days after Richard's death, the U.S. Supreme Court announced it would hear a challenge to the way executions like Richard's are carried out. That case will be heard Monday.

The hearing will weigh whether the nearly universal three-chemical mix used to execute condemned prisoners is constitutional. The court's decision could determine whether a de facto moratorium on the death penalty continues.

Since the court announced it would take the case, it has intervened several times to grant last-minute stays. States across the country have followed suit.

Monday's arguments in front of the high court is the most significant death penalty case in years. The issue presented does not address the constitutionality of the death penalty itself, but rather the method by which nearly all executions in the U.S. are currently performed.

The case, Baze v. Rees, comes out of Kentucky, which is one of 36 states that provides for capital punishment as its most severe criminal sentence. New Jersey's governor last month signed legislation prohibiting the death penalty, and the state has since cleared its death row, returning prisoners to general lock-up.

Ralph Baze was convicted of the 1992 murders of Powell County Sheriff Steve Bennett and his Deputy Sheriff Arthur Briscoe. Thomas Bowling was convicted of killing Eddie and Tina Early after a traffic accident. He also shot and wounded their two-year-old son.

In its brief to the court, lawyers for Baze and Bowling say the three-chemical cocktail and the protocols Kentucky uses to administer the death sentence are cruel and unusual and as such are unconstitutional. They argue "a state...violates the Eighth Amendment when its execution procedures create a significant and unnecessary risk of inflicting severe pain that could be prevented by the adoption of reasonable safeguards."

The three-chemical mix was developed in Oklahoma in 1977 and first used five years later in Texas. The first chemical is sodium thiopental — a barbiturate used to anesthetize the inmate. The second chemical is pancuronium bromide, which is used to inflict paralysis. Finally, an injection of potassium chloride causes cardiac arrest and death.

The last chemical is considered to be incredibly painful. The lawyers argue "if the intended dose of thiopental is not injected successfully, or does not bring about general anesthesia, the inmate will experience both the terror and agony of conscious suffocation and the excruciating pain caused by the potassium, but will appear peaceful and unconscious to observers." They further argue to the court that only sodium thiopental is necessary to carry out a humane execution.

A trial court in Kentucky rejected the arguments made on behalf of the murderers, as did the Kentucky Supreme Court in a unanimous ruling. Kentucky's argument to the court draws upon these lower court rulings and asserts that its execution method does not present a "substantial risk" of an Eighth Amendment violation.

The state argued this standard is the proper one for courts to use, suggesting the alternative of "unnecessary risk" is too broad. "[T]he 'substantial risk' standard applied by the Supreme Court of Kentucky provides the proper balance between the need to control the proliferation of insubstantial litigation and the interests of death row inmates."

The history of the death penalty in the United States is long and has evolved significantly over the decades. Hanging was the method of choice in the 19th century but a move to supposedly more humane executions during the 20th century brought with it the emergence of the electric chair and gas chamber. By the end of the 20th century and as it stands today, lethal injections are considered the humane standard. But the three-chemical method used by 35 of the 36 death row states — Nebraska is the only state where electrocution is the sole method of death — is what the court may determine is cruel and unusual.

Richard was the 42nd and last person executed in 2007. That figure represents the fewest number of executions since 1994. According to the non-profit Death Penalty Information Center, he was the 1,099th person executed since the Supreme Court reinstated the death penalty in 1976. Kentucky has only executed two people in that time.

Because of the significant interest in the case, the court has made the unusual, though not unprecedented, decision to release an audio broadcast of the oral arguments immediately after its conclusion.