The first execution using nitrogen gas in the United States has put the spotlight on the country's use of capital punishment.

Kenneth Eugene Smith, 58, convicted of a 1988 murder-for-hire, was put to death in the southern state of Alabama on Thursday.

Executions in the United States are generally carried out by lethal injection, but Smith was the first person to be executed using nitrogen gas.

Here are key facts about the death penalty in the United States:

- Number of US executions declining -

There were 24 executions in the United States in 2023, six more than in 2022.

While more people were put to death last year than the previous year, the number of executions annually in the United States has been declining.

Last year was the ninth consecutive year with fewer than 30 executions, according to the annual report of the Death Penalty Information Center (DPIC).

And fewer states are putting inmates to death.

In 2023, executions were carried out in only five states -- Alabama (2), Florida (6), Missouri (4), Oklahoma (4) and Texas (8).

- Death sentences also declining -

Only seven states sentenced people to death in 2023, according to the DPIC, and the number of executions (24) exceeded the number of new death sentences (21) for the first time.

The DPIC said many of the 24 people executed last year suffered from "significant vulnerabilities" such as mental illness, intellectual disability, childhood trauma, neglect or abuse.

"Many likely would not have been sentenced to death if tried today," it said.

- Public opinion -

According to a recent Gallup poll, 53 percent of Americans support the death penalty for someone convicted of murder, the lowest level since 1972.

Capital punishment has been abolished in 23 US states, while the governors of six others -- Arizona, California, Ohio, Oregon, Pennsylvania and Tennessee -- have put a hold on its use.

- Methods -

The primary method of execution in the United States remains lethal injection but three states have authorized the use of nitrogen gas, a method the United Nations has likened to "torture."

Execution by nitrogen hypoxia involves placing a mask over the condemned inmate's face and then administering pure nitrogen gas, depriving the body of oxygen.

Five states have approved the use of firing squads, but the last execution carried out using this method was in 2010 in the western state of Utah.

- Death Row -

There were 2,331 prisoners on Death Row in the United States as of January 1, 2023, according to the DPIC.

California, which has not carried out an execution since 2006, had the largest Death Row population with 665 inmates, followed by Florida (313) and Texas (192).