WASHINGTON (CNN) -- The U.S. Supreme Court appeared divided along ideological lines Monday over whether lethal-injection execution methods in about three dozen states are being properly and humanely applied.
The high court's involvement in the case has halted executions at least until the summer.
At issue are the standards to determine whether the most common method of capital punishment can cause excruciating pain, violating the Constitution's ban on "cruel and unusual punishment."
"I am terribly troubled," said Justice John Paul Stevens, referring to the use of one type of drug in the lethal injection mixture that is designed to paralyze a condemned prisoner.
"If it is properly administered, would you have a case here?" asked Justice Anthony Kennedy, whose vote could swing a ruling to either side.
The justices have never directly addressed the fundamental question over the use of the chemical "cocktail" of drugs used to execute convicted killers. All but one of the states that perform such executions use the three-drug mixture at issue in this appeal.
Kentucky inmates Ralph Baze and Clyde Bowling Jr. brought suit in federal court three years ago, questioning that state's chemical mixture and the procedures used to administer it. Watch how the drugs work »
They claim the first drug -- sodium thiopental -- which renders the prisoner unconscious, wears off too quickly. Some prisoners are awake and able to feel pain as the procedure continues, the condemned men contend.
The second drug -- pancuronium bromide -- paralyzes all muscle movement, then prevents the condemned person from speaking out and expressing awareness of the pain, according to the men's attorneys.
The third drug, potassium chloride, which induces cardiac arrest, is "excruciatingly painful in a conscious person," the inmates and lawyers say in their court papers.
Baze, 52, admits killing Powell County Sheriff Steve Bennett and Deputy Arthur Briscoe in 1992 while the lawmen were trying to serve him with arrest warrants.
Bowling was convicted of killing Edward and Tina Earley in Louisville in 1990. Their 2-year-old son was wounded in the attack in the parking lot of the couple's dry-cleaning business. Bowling's attorneys have alleged prosecutorial misconduct in the case and say their client has a low IQ.
Officials in Kentucky say their procedures are in line with other states, and a doctor is on site and available to provide any pre-execution medical care to the inmate.
A Texas inmate was put to death the day the high court accepted the Kentucky cases. Since then, the justices have stepped in and imposed a de facto moratorium nationwide. Depending on how the justices rule, capital punishment could effectively be delayed for a year or more.
Twenty-six people were executed last year, the lowest total in more than a decade.
In oral arguments, the justices debated the threshold when execution procedures become unconstitutional. No consensus was reached, although both sides of this politically charged issue are hoping for clear guidelines from the Supreme Court.
Baze and Bowling's attorney, Donald Verrilli, said inmates run "the risk of a cruel and inhumane" death from a flawed execution method. Watch what the lawyers had to say »
"Of course, there is risk of human error," said Justice Stephen Breyer. "But can we say there is a more serious problem here than other execution methods?"
Verrilli suggested a single drug -- a strong barbiturate used on animals -- might be a more preferable alternative.
"It's never been tried on humans," said Chief Justice John Roberts. "Can we establish it doesn't cause pain?"
Verrilli replied the second drug of the current mixture is "just an anti-convulsant" designed to ensure the audience witnessing the execution is not personally in "discomfort" seeing a dying person's body reacting to the drugs by twitching uncontrollably.
But using this drug "also enhances the dignity of the inmate," Roberts asserted. He also said he wondered whether a fresh round of legal appeals would start if a single drug was used to perform executions.
Stevens reiterated his concern that the second drug "seems totally unnecessary."
Arguing the other side for the state, Roy Englert said Kentucky has "excellent" procedures and safeguards, including trained technicians who specialize in inserting needles in veins, and monthly practice sessions at the correctional facility housing death row. He also said technicians are always "in close proximity" to the gurney where the inmate is placed.
Justice Ruth Bader Ginbsurg said she wondered why technicians were not next to the prisoner to ensure there were no last-minute problems. Englert explained it was to preserve the anonymity of the technicians, who might be subject to harassment or threats as the official executioners.
Stevens and Justice David Souter suggested the high court postpone a decision on the application of lethal injection until lower courts can fully investigate alternative methods of death that might be more acceptable.
Stevens told Englert that Kentucky's procedures with the three-drug mixture were "very persuasive in your favor" but wondered whether other states could apply them without risk of lengthy legal challenges.
Justice Anthony Scalia said he worried about endless litigation if the issue is not settled soon. "This never ends," he said.
Various states and judges have applied different standards over whether an inmate can make a challenge to the method of execution.
In September, U.S. District Judge Judge Aleta Trauger stopped a planned execution in Tennessee, saying correction officials' new method of execution in that state "presents a substantial risk of unnecessary pain."
And in 2005, then-Florida Gov. Jeb Bush issued a moratorium in his state after complaints over the length of time needed to execute prisoner Angel Nieves Diaz. A two-year review was completed, but executions were halted further by the high court's current review of the issue.
The Supreme Court has not ruled directly on the "cruel and unusual" aspect of lethal injection, but it did conclude in 2006 that prisoners can make last-ditch legal challenges to the method of execution, using claims that they would suffer a painful death.
"The question before the court is how much risk of unnecessary pain is too much risk of unnecessary pain under the Constitution," said Edward Lazarus, an appellate attorney and Supreme Court legal analyst.
"There is evidence to show that this is somewhat of an inhumane way to approach this, but there's going to be a group of justices who really think we shouldn't be in the business of micromanaging the death penalty," he added.
A ruling in the case is expected by late June. E-mail to a friend