HUNTSVILLE - The illuminated clock set in red-brick facade of the Walls Unit may be the most dreaded timepiece in Texas. Minute by minute, it ticks away dreary years behind bars. On some days - 15 times last year, 40 times in 2000 - its black hands signal another criminal justice milestone.

Six o'clock, the hands say. Another killer will be dead and gone.

Kimberly McCarthy on June 26 became the 500th Texas inmate to be executed since the state re-activated the death penalty in 1976. Texas leads the nation's 33 death penalty states in executions, killing more than the next five most active states combined.

Virginia, with 110 executions, places second.

Texas history: Notable executions

McCarthy, 52, condemned for the 1997 murder-robbery of a 70-year-old Dallas County woman, was strapped to a gurney in a room deep within the 164-year-old prison. She was injected with a lethal dose of a drug commonly used to euthanize cats and dogs.

McCarthy, a one-time occupational therapist and home health care worker, was the fourth woman in Texas executed by injection.

More Information Executions in Texas Executions in Texas date to 1819 when a man was hanged in Galveston for piracy. Since then, offenders have been put to death by hanging, under the authority of individual counties, and by electrocution and lethal injection, under authority of the state of Texas: Hangings (1819-1923): 390 Electrocutions (1924-1964): 361 Lethal injections (1976-2013): 499 Sources: The ESPY File, Texas Department of Criminal Justice

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Supported by law since Texas' earliest days, executions like hers remain at the heart of a raging dispute pitting most Texans against an array of death penalty opponents here and abroad.

Death penalty: Before Texas got the needle, counties got the rope

Issues of justice arise as prisoners, including some on death row, are shown years after their convictions to be innocent, and scrutiny is directed to cases of executed men whose convictions were based on questionable investigations. Increasingly, the Texas Department of Criminal Justice has found itself challenged to carry out executions as death penalty opponents pressure drug makers to stop sales to executioners.

Nonetheless, capital punishment retains strong support in Texas. Last year, a University of Texas/Texas Tribune poll found almost three-fourths of respondents favored executions. Nationally, a Gallup Poll found 63 percent endorsed capital punishment.

Killers from Harris County, Texas' most populous county, fill the Polunksy Unit's death row. Since 1982, 118 Harris County killers have been executed; 100 remain at the Livingston prison. Fifty Dallas County killers have been put to death; 37 from Bexar, the county encompassing San Antonio.

Arguments both ways

Typical of the extremes in the death penalty debate are Ray Hunt, the Houston Police Officers Union president, who calls for expanding the death penalty to cases of brutal child abuse, and Anthony Graves, who was condemned for murders he did not commit.

"There's no doubt in my mind," Hunt says when asked if executions have made Texas safer. "The 500 people who are executed - they have no opportunity to brutally murder again."

Counters Graves, who was released from prison in 2010 after prosecutors admitted he had been wrongly convicted in the August 1992 killings of six Somerville residents: "For me, the death penalty is a slap in the face. I spent 18 years in prison, 12 of them on death row with two execution dates, and it doesn't even slow down. It says to me: 'Your life has no value.' "

An avid death penalty supporter, Gov. Rick Perry has defied the United Nations in refusing to intervene in the executions of foreign nationals. In a 2011 presidential debate that brought cheers from Republican supporters, Perry remarked that he "never struggled" with allowing executions to occur.

"In the state of Texas," he said, "if you come to our state and you kill one of our children, you kill a police officer, you're involved with another crime and you kill one of our citizens, you will face the ultimate justice …"

Former Gov. Mark White, whose 1983-87 tenure saw the execution of 19 killers, is less certain.

"Right now," he says, "our power to execute is very flawed. … It's a serious problem that needs to be re-examined from ground zero. … I still support the death penalty, but I don't support it for innocent people."

Pulling the switch

Known executions date to 1819, a period in which Texas was a Spanish possession. Six were hanged during Texas' brief existence as a republic, and hanging - carried out at the county level - remained the primary manner of execution in the state until 1923.

In that year, the authority to execute - employing the new-to-Texas electric chair - was assumed by the state, in part, Texas Prison Museum staff says, because public hangings attracted unruly crowds.

Six African-American men were electrocuted in "Old Sparky" on Feb. 8, 1924, The Walls' inaugural day as the official Texas death house. Warden R.F. Coleman resigned rather than pull the switch.

"It just couldn't be done, boys," he told reporters. "A warden can't be a warden and a killer, too. The penitentiary is a place to reform a man, not to kill him."

His successor had fewer qualms.

An additional 355 convicted killers were electrocuted before the U.S. Supreme Court's 1973 Furman v. Georgia ruling brought capital punishment to a nationwide halt. Sixty-three of them were from Harris County.

The Furman ruling and those in two related cases found the death penalty cruel and unusual, in part because of the lack of uniform criteria under which it could be imposed. Texas and other death penalty states rewrote their laws. The high court approved the changes in its 1976 Gregg v. Georgia decision, and executions resumed.

Texas' revamped law required jurors to answer two questions: Does the killer present a continuing threat to society? Are there mitigating factors regarding character and background that warrant a life sentence?

Texas' first post-Gregg execution, that of Charlie Brooks, condemned for the abduction and murder of a Fort Worth auto mechanic, was carried out on Dec. 7, 1982. Brooks, 40, also was the first killer executed by a lethal injection of sodium thiopental, an anesthetic; pancuronium bromide, a muscle relaxant; and potassium chloride to stop the heart.

International attention

Slowly Texas' revamped death penalty gained momentum. By the late 1990s, executions topped 35 a year. A record 40 killers were put to death in 2000. In June 2000, Texas executed Gary Graham, convicted of a 1981 Houston supermarket robbery-murder.

In a case that drew international attention, prosecutors relied heavily on the testimony of an eyewitness, who said she saw Graham accost a shopper in a dimly lighted parking lot. Graham claimed innocence but admitted that he had committed a string of violent crimes. Civil rights leaders Coretta Scott King, Al Sharpton and the Rev. Jesse Jackson rallied to his cause, but to no avail.

From the gurney, Graham denounced his execution as "a lynching."

David Atwood, founder of the Texas Coalition to Abolish the Death Penalty, found Graham convincing.

"I very strongly believed Graham had not committed the crime," the Houston activist says. "… I think there's pretty strong evidence - it's hard to say it's 100 percent - that in three or four instances we could have executed someone who's innocent."

Cases concerning him include those of Ruben Cantu, executed in 1993 for a San Antonio robbery-murder the former district attorney now believes he probably did not commit; Carlos DeLuna, put to death in 1989 for a Corpus Christi robbery-murder, the investigation of which Columbia University law students critique as flawed; and Cameron Willingham, executed in 2004 for the murder of his three young children in a Corsicana house fire.

The reliability of two arson investigations key to Willingham's conviction was questioned in three reviews by modern arson experts.

"Guided by scientific advancement, we know that investigation guidelines for arson that we used for years were phony science," says White, who publicly has spoken on Willingham's behalf. "… I think we've executed some (legally) 'not guilty' people. I can't tell you for certain they were innocent."

'Actual innocence' rare

Texas, says death penalty advocate Dudley Sharp, likely has executed an innocent person since it gained statehood in 1845. Even so, "based on the evidence, none of the commonly referred-to cases apply," he adds.

"We have to rely upon the courts for such a determination, for which no innocent executed has been found," Sharp says.

Death sentences, he says, continue to be warranted: "The death penalty is needed for the same reason all criminal sanctions are, that being a just and appropriate sanction, which is proportional to the crime."

Though controversial for decades, the state's death penalty in recent years has attracted unprecedented scrutiny.

Death penalty opponents argue that as many as 12 people have been wrongly convicted and later cleared of capital murders since executions resumed. Sharp dismisses such claims as "fraud," insisting few of those released from death row were convincingly shown to be innocent.

Still, some such convictions - such as Graves' - have been proved false. In 2004, Ernest Willis was freed from death row after serving 17 years for a West Texas arson fire murder. After a federal court ordered that Willis be granted a new trial or released, Pecos County District Attorney Ori White determined "the facts in the case exonerate Mr. Willis."

Graves and Willis are the only exonerated capital killers to receive state compensation for the time they spent on death row, a designation that requires a finding of "actual innocence."

Life without parole

David Weeks, the Walker County district attorney and a former president of the Texas District and County Attorneys Association, says such failings largely occurred in "old cases."

"I think that to a large extent, it's not the same old world, not the same old situation it was," he says. "We've done quite a lot to make sure the process works. Can it be improved more? Probably so, but we have to realize how far we've come."

Now, Weeks argues, lawyers appointed to represent accused killers are "well qualified," and DNA investigation tools work to lessen the chance of false arrests.

Changes to the Texas death sentence have come from inside and outside the state.

In 2005, the Texas Legislature for the first time enabled juries to sentence capital killers to life without parole. Since then, 535 death penalty-eligible killers, 144 from Harris County, have been assessed lifelong incarceration. No death sentences were handed down in Harris County in 2012.

Fifteen Texas killers were executed in 2012; McCarthy will be the eighth this year.

Down to one drug

Since 2011, British activists twice have succeeded in persuading makers of drugs used in Texas' lethal injections to stop providing them to the state. Since mid-2012, Texas has used a single drug, pentobarbital, for executions.

Someday, Atwood says, Texans will look back on the coming 500th execution as a historic low point. Prison activist Ray Hill believes that day may come sooner than expected.

"I think the U.S. Supreme Court will strike down capital punishment when the 26th state decides to stop executing criminals," he says. "Then it becomes cruel and suddenly unusual based on the changing standards test."

Maryland in May became the latest state to repeal capital punishment.