Death penalty in the US -- by the numbers

The US Supreme Court is due to rule on a drug cocktail used in some lethal injections to kill death row convicts, amid growing pressure to abolish capital punishment.

Here are facts on the current state of the death penalty and lethal injection in the United States.

Death penalty status at state level

A total of 18 US states and the US capital Washington have abolished the death penalty, including six since 2007: Alaska (1957), Connecticut (2012), Hawaii (1957), Illinois (2011), Iowa (1965), Maine (1887), Maryland (2013), Massachusetts (1984), Michigan (1846), Minnesota (1911), New Jersey (2007), New Mexico (2009), New York (2007), North Dakota (1973), Rhode Island (1984), Vermont (1964), West Virginia (1965), Wisconsin (1853) and the District of Columbia, home to the capital Washington (1981).

The New Mexico and Connecticut appeals were not retroactive, meaning that convicts on death row at the time remained there.

Nebraska is expected to soon join the list. Delaware's senate has voted to repeal the death penalty, but a vote is still pending in the state house of representatives.

Of the 32 states where the death penalty is still on the books, 11 have renounced putting convicts to death, meaning that in practice, 29 of the 50 US states no longer use the death penalty.

Prisoners on death row

On January 1, 3,019 prisoners were on death row, compared to 420 in 1976, when the death penalty was reinstated in the United States. At its peak in 2000, there were 3,593 prisoners on death row, according to the Death Penalty Information Center.

Fewer executions

Of the 1,407 executions performed in the country since 1976, 13 have taken place so far this year.

There have been fewer executions over the past 20 years. In all of 2014, 35 people were executed, compared to 39 in 2013, 43 in 2012 and 2011, 46 in 2010 and 52 in 2009, according to DPIC data. There were 98 executions in 1999, a record.

Last year, 80 percent of executions took place in the most active death penalty states: Texas, Missouri and Florida.

The US military has not executed prisoners in five decades. The federal government has requested the death penalty for the Boston Marathon bomber but has not executed prisoners since 2003, the last of three executions in four decades.

Wrongfully convicted

A total of 152 prisoners have so far been found innocent and freed from death row in 26 states, according to the DPIC.

Florida is the state with the most sentencing errors in this regard, with 25 prisoners found innocent, ahead of Illinois with 20 and Texas with 12.

Popular opinion sways

The death penalty is more unpopular than ever, with just 56 percent of those polled supporting it, while 38 are against, according to the latest Pew survey. It has been dropping since 1996, when 78 percent of Americans backed the punishment. In 2011, it stood at 62 percent.

Lethal injection impasse

A shortage of lethal injection drugs is giving a major headache to death penalty states, after pharmacies refused to supply or export barbiturates for executions.

Recent executions have triggered international outcry after prisoners appeared to suffer during botched killings in Arizona, Ohio and Oklahoma.

Old methods return

Faced with problems in trying to use lethal injections, some states have revived old methods of execution. Utah backed the use of firing squads, Tennessee is using the electric chair, while Oklahoma has turned to nitrogen gas chambers.