Ruben Cantu is long gone, executed by Texas authorities in 1993 after he was convicted of murdering a man during a San Antonio robbery when he was 17 years old. To the end, Cantu insisted he had been framed, and now his co-defendant and the sole surviving witness both say he was telling the truth.

A state legislator called for an investigation this week as prosecutors moved to study the 20-year-old case. Opponents of the death penalty suspect that Cantu may be what they have long expected to find: an innocent person put to death. Houston law professor David Dow said the case shows that "we make mistakes in death penalty cases, too."

The nation's 1,000th execution since the Supreme Court reinstated the death penalty in 1976 was scheduled to occur Friday morning, barring a last-minute stay. The expected execution of Kenneth Boyd in North Carolina for murdering his wife and her father comes at a time of growing misgivings over the death penalty, as reflected in jury verdicts, opinion polls and the actions of courts and state legislatures.

Death sentences have declined to their lowest level in three decades, with juries sentencing 125 people to death last year, compared with an average of 290 per year in the 1990s. The number of inmates executed last year was the lowest since 1996, and the Supreme Court has twice in the past three years limited who can be punished with death.

In Virginia, which has executed more people since 1976 than any state but Texas, Gov. Mark R. Warner (D) commuted the death sentence of Robin M. Lovitt this week because the state had thrown out what may have been conclusive evidence, making this the first year since 1983 that Virginia will not have had an execution.

In Maryland, Cardinal William H. Keeler, the archbishop of Baltimore, prayed with Wesley E. Baker this week and said he would appeal to Gov. Robert L. Ehrlich Jr. (R) to commute his death sentence to life without parole. Baker is on death row for murdering teacher's aide Jane Tyson during a robbery outside a Catonsville mall.

Public opinion polls show that nearly two-thirds of Americans support the death penalty, but that is a significant drop from the peak, in 1994, when 80 percent of respondents told Gallup pollsters they were in favor of capital punishment. When asked if they would endorse executions if the alternative sentence of life without parole were available, support fell to 50 percent.

Amid the refinement of DNA techniques and the sporadic release of inmates from death row because of uncertain guilt, a growing number of people tell pollsters they believe that innocent prisoners have been executed. Although the majority of cases over the past three decades have been upheld, legal errors and sometimes poor defense work revealed during layers of appeals have convinced many Americans that the system is imperfect.

"There's a skepticism about the accuracy of the system and, to some degree, the fairness," said Richard C. Dieter, executive director of the Death Penalty Information Center. "It's not quite the ticket to the statehouse if you promise to execute more and more and speed it up. You have religious leaders voicing concerns. You have conservatives. The lines aren't as clear as they were before."

One place to observe the recent push and pull is Illinois, where outgoing Gov. George Ryan (R) commuted the sentences of the state's 167 death row inmates in 2003, calling the state's death penalty "arbitrary and capricious."

A 2000 moratorium on executions continues in Illinois, but 10 more defendants have been sentenced to die since Ryan acted. At the same time, the state legislature has reformed death penalty rules, provided more money for defense lawyers and required police to videotape the questioning of homicide suspects.

"The playing field has been leveled. We think that's fine," said Paul A. Logli, a prosecutor in Rockford, Ill., who said he thinks Ryan's clemency move was a mistake. "In one broad brush, he swept death row, even of those people who had never asserted their innocence. I believe the better choice is that governors should do it on a case-by-case basis."

Logli, who is also president of the National District Attorneys Association, said prosecutors in the 38 states with a death penalty "by and large believe in it as a deterrent and believe it should be used wisely, sparingly."

The New York legislature this year stopped short of renewing the state's death penalty law, which a court had declared invalid. North Carolina, where condemned prisoner Alan Gell was acquitted in a retrial with the help of evidence that was initially suppressed, created a commission to study how the death penalty operates. California, home to the nation's largest death row, with 648 inmates, did likewise.

Elsewhere, legislation on the desk of Wisconsin Gov. Jim Doyle (D) is designed to improve witness-identification procedures, require electronic recording of interrogations and ensure the preservation of DNA evidence. If the interviews are not recorded, juries are to be told that police violated the law.

Texas has executed more than one-third of the men and women put to death since 1976, as well as 19 of the 55 inmates executed this year. With no more executions scheduled there this month, the state's total this year will fall well below its eight-year average of 28.

Jordan M. Steiker, a death penalty expert at the University of Texas, attributes the slowdown largely to federal court concerns, notably the 2002 Supreme Court decision to bar the execution of the mentally retarded and this year's ruling prohibiting the execution of juveniles.

Gov. Rick Perry (R) has signed a law that offers juries a chance to sentence capital defendants to life without parole. He also created a Criminal Justice Advisory Council, which has a committee that will focus on the death penalty.

State Rep. Elliott Naishtat (D), a death penalty opponent from Austin, said: "We are seeing a mood change. Legislative and executive branches are responding to a clear change in public confidence."

A poll conducted by Rice University sociologist Stephen L. Klineberg found that even in Harris County, where more defendants are sentenced to die than anywhere else in the country, support for the death penalty fell from 68 percent in 1999 to 60 percent this year. In response to a separate question, 53 percent favored life without parole as an alternative.

The latest survey was conducted before last month's Houston Chronicle series on Ruben Cantu.

Prosecutors presented no physical evidence linking Cantu to the 1984 nighttime attack in San Antonio. Nor did they call the 15-year-old co-defendant, who had pleaded guilty to the lesser crime of robbery and identified Cantu, an admitted thief, as his partner. The principal witness was Juan Moreno, the wounded worker.

The jury did not believe Cantu, who said he had been in another city that night, and ordered him to die. He was executed on Aug. 24, 1993.

The Chronicle found numerous gaps and flaws in the investigation. Further, Moreno told the newspaper that he had accused Cantu under police pressure.

Cantu's co-defendant, David Garza, signed a sworn statement saying he lied when he told police that Cantu was with him that night, the newspaper reported. He now says he knew who the true killer was, but honored a neighborhood code of silence in not naming him.

"You've got a 17-year-old who went to his grave for something he didn't do," Garza said, adding that he was speaking now because of a guilty conscience. He said that as Cantu's execution date neared, he wrote Cantu's unpaid appellate lawyer from prison to say the case was "real messed up" and added: "Hope to hear from you real soon."

The lawyer told the Chronicle that she doubted the worth of Garza's comments and did not inquire further. Roy R. Barrera Jr., the trial judge, told the newspaper that he was shocked that no defense lawyer interviewed Moreno or compelled Garza to testify during the appeal. But he also said he believes Garza is now lying "because he has nothing better to do and wants to put everybody on a guilt trip."

Guilty or innocent, Cantu could not have been executed if he were prosecuted now. According to this year's Supreme Court ruling, he was too young at the time of the crime.