Kenneth Smith, son of killer Kenneth Lee Boyd sits with his daughter Courtney, 9, in the visitor center at Central prison in Raleigh, North Carolina

Convicted killer Kenneth Boyd has become the 1,000 person to be executed in the United States since the death penalty was reinstated in 1976.

Boyd, 57, received a lethal injection and was pronounced dead early Friday, in a prison in the southern state of North Carolina.

He had been convicted for the 1988 murder of his estranged wife and her father.

Prison security stands guard outside Central Prison in Raleigh, N.C., Thursday night

Boyd's execution was carried out after North Carolina Governor Mike Easley and the U.S. Supreme Court denied his final pleas for clemency.

In 1977, convicted killer Gary Gilmore became the first person put to death in the United States after the U.S. Supreme Court ended a 10-year moratorium on executions.

Capital punishment was blocked for a time because the court ruled that state death penalty statutes were too arbitrary. Many states rewrote their laws, and today, more than 3400 prisoners are awaiting execution.

But since 1973, 122 prisoners have been freed from death row, mostly due to the increasing use of DNA evidence.

Death penalty opponents argue that the U.S. criminal justice system makes mistakes, and that some inmates may have been sentenced to death for crimes they did not commit. Susan Karamanian is a legal scholar at George Washington University Law School in Washington D.C.

"We have heard year after year, month after month, day after day, horror stories about people who have been tried and convicted in a system that is not fair," she said.

Ms. Karamanian says the public debate about capital punishment used to be about whether it deterred crime. But she says public opinion is now shifting to question whether the U.S. justice system adequately protects innocent people from execution.

An October Gallup poll found that about two-thirds of Americans still support the death penalty. That compares with a high of 80 percent of Americans in 1994. And when respondents are given a choice of life in prison without parole, support for the death penalty falls to just 50 percent.

Ms. Karamanian attributes the change in opinion partly to a trend, beginning in the 1990s, of highly qualified lawyers representing inmates on death row and raising issues not addressed in the past.

"When you have prominent lawyers, members of the Bar [American Bar Association], getting active in these cases, one, you have a better standard of representation, but two, word is getting out," she said.

For example, Virginia Governor Mark Warner recently blocked the execution of Robin Lovitt, who would have been the 1,000h person to be executed in the United States. One of Mr. Lovitt's attorneys was Kenneth Starr, who led the special investigation into President Bill Clinton in the 1990s.

Mr. Starr successfully argued that Mr. Lovitt deserved clemency because a Virginia court clerk had wrongly discarded DNA evidence. Instead of being executed, Mr. Lovitt will now spend his life in prison for killing a night manager in a pool hall.

Analysts say another factor influencing public opinion is the growing opposition to capital punishment among U.S. Catholics.

In November, the U.S. Conference of Catholic Bishops overwhelmingly approved a new statement opposing the death penalty. The bishops' first comprehensive statement on capital punishment in 25 years said that, "the sanction of death violates respect for human life and dignity."

Still, advocates of the death penalty argue that abolishing capital punishment would hurt crime victims and their families.

Michael Paranzino is the head of Throw Away the Key, a group in Maryland that advocates tough sentences for criminals.

"The idea that we would mourn the 1,000th murderer that died when there are 600,000 families who will never recover - that's where I think this debate has to be focused -- the lasting harm to entire families when people are murdered," he said.

Regardless of how the American public feels about capital punishment, statistics from the Death Penalty Information Center in Washington D.C. show that death sentences nationwide have dropped by 50 percent since the late 1990s and executions carried out have declined by 40 percent.