SAN ANTONIO, Texas (Reuters) - Executions in the United States are decreasing due to concerns about costs, flawed prosecutions and shortages of drugs needed to carry out lethal injections, the Death Penalty Information Center said in a report on Thursday.

There have been 39 executions carried out in the United States in 2013, down from 43 executions in each of the past two years, the group - a well-regarded source of death penalty data - said in its annual report.

The number of people sent to the death chamber has been on a general decline since 1999, when 98 people were executed.

“The realization that mistakes can be made, and innocent people have been freed who could have been executed - that causes jurors to hesitate. Prosecutors know it is harder to get a death sentence,” the center’s executive director, Richard Dieter, told Reuters.

The last planned execution in the United States for 2013 took place on Tuesday in Oklahoma, when the state killed by lethal injection a man convicted of stabbing and beating a horse trainer to death in a case of mistaken identity.

The center said so far this year 80 individuals were sentenced to death, fewer than the 315 death sentences meted out in both 1994 and 1996, representing recent historical high numbers. The 2013 tally was also the lowest for a single year since the U.S. Supreme Court reinstated capital punishment in 1976.

About 60 percent of Americans say they favor the death penalty for convicted murderers, polling group Gallup said in October.

Gallup said the number marks the lowest level of support it has measured since November 1972, when 57 percent were in favor. Death penalty support peaked at 80 percent in 1994.

Dieter said the decline in death sentences has been accompanied by an increase in sentences of life imprisonment without parole.

“Jurors like that option,” he said.

BEING SELECTIVE

Prosecutors who support the death penalty, like Susan Reed, the district attorney in San Antonio, said they consider costs in pursuing a capital punishment conviction, which usually involves years of expensive mandatory appeals.

“We are now very selective in what we choose to go after as death penalty cases, instead of deciding that every single murder that we try will be a capital case,” Reed told Reuters.

Several states have had trouble procuring drugs for lethal injections because pharmaceutical companies have shied away from direct sales, not wanting to be associated with executions.

Six states have repealed capital punishment in the last seven years, the center said, with Maryland being the latest.

Maryland Governor Martin O’Malley said in May when he signed the measure: “Over the longer arc of history, I think you’ll see more and more states repeal the death penalty. It’s wasteful. It’s ineffective. It doesn’t work to reduce violent crime.”

New technology, embraced by the advocates of those who say they were wrongly convicted, has led to 311 post-conviction DNA exonerations in the United States since 1989, according to the Innocence Project.

Of those exonerated, 18 served time on death row and an additional 16 were charged with capital crimes but did not receive a death sentence.

But John Malcolm, director of the Edwin Meese III Center for Legal and Judicial Studies at the Heritage Foundation and a former federal prosecutor, said the death penalty has a place in the judicial process.

“When presented with the facts in individual cases .., support for the death penalty goes up dramatically, and roughly half of Americans say that the death penalty isn’t imposed often enough,” Malcolm told Reuters.

“Their views are entitled to respect.”