ST. LOUIS (AP) — A bungled execution Tuesday in Oklahoma provides death penalty opponents with a fresh, startling example of how lethal injections can go wrong. But the odds of successfully challenging the nation’s main form of capital punishment will probably hinge on exactly what caused the inmate’s apparent agony. If four-time convicted felon Clayton Lockett suffered because of a collapsed vein or improperly inserted IV, the legal landscape might not change much. If the execution drugs or the secrecy surrounding them played a role, defense attorneys for other prisoners could have powerful new evidence to press the U.S. Supreme Court to get involved, legal experts say.Attorneys for some death-row inmates began planning new appeals or updating existing cases based on events in Oklahoma. Many called for moratoriums and independent investigations.“Every prison is saying, ‘We have it under control, trust us,’” said Texas attorney Maurie Levin, who spent Wednesday preparing new briefs questioning that state’s execution practices. “This just underscores in bold that we can’t trust them, and prisons have to be accountable to the public and transparent in the method by which they carry out executions.”The 38-year-old Lockett, convicted of shooting a woman and watching as two accomplices buried her alive, was declared unconscious 10 minutes after the first of three drugs was administered Tuesday. Three minutes later, he began breathing heavily, writhing, clenching his teeth and straining to lift his head.Authorities halted the execution, but Lockett died of a suspected heart attack more than 40 minutes after the process began.An autopsy was conducted Wednesday to determine his precise cause of death, and Oklahoma Gov. Mary Fallin named a member of her cabinet to lead a review of the state’s execution procedures.The White House said the execution fell short of the humane standards required.Courts, including the Supreme Court, have been reluctant to halt executions over arguments that they violate an inmate’s constitutional guarantee against cruel and unusual punishment. In four rulings over the past 135 years, the Supreme Court has upheld the use of the firing squad (1879), the electric chair (1890), the ability of a state to try to execute a condemned inmate by electrocution again after a first attempt failed (1947) and lethal injection (2008).The Constitution “does not demand the avoidance of all risk of pain in carrying out executions,” Chief Justice John Roberts said in the court’s 2008 decision upholding Kentucky’s lethal injection system.Still, a minority of the high court has shown some recent trepidation about the secrecy of the process used by many states.Many states — Oklahoma, Texas and Missouri among them — purchase execution drugs from lightly regulated compounding pharmacies and refuse to name the supplier, whether the drug has been tested, even who is part of the execution team.In February, three justices — two short of the required five — said they would have blocked the execution of Michael Anthony Taylor in Missouri. A month later, four justices fell one vote short of blocking the execution of another Missouri inmate, Jeffrey Ferguson. They offered no explanation for their vote.If Tuesday’s problems are traced to a collapsed vein, the high court “probably won’t feel a lot more pressure to step in,” said Thomas Goldstein, an experienced Supreme Court lawyer who also has represented death-row inmates. But if the injection chemicals themselves and the state’s secrecy emerge as important factors, “there will be great pressure for them to hear a case and require transparency.”Madeline Cohen represents Charles Warner, an Oklahoma inmate who was scheduled to be executed Tuesday just hours after Lockett. She said she plans new appeals on behalf of Warner, whose execution was postponed for at least two weeks.She also is pressing for an independent investigation of Lockett’s death, including examination of his remains by an independent pathologist.In Missouri, convicted killer Russell Bucklew is scheduled to die May 21. His attorney, Cheryl Pilate, said she plans to file new appeals next week seeking to halt the execution or at least delay it until the state’s procedures “are subject to full disclosure.”The potential for something to go wrong is escalated for Bucklew, Pilate said, because he suffers from a lifelong medical condition that has left his blood vessels malformed and weakened. It’s so bad that he often bleeds from the eyes, Pilate said.“Executions are not medical acts,” Pilate said. “They are experiments conducted on human subjects with no accountability or oversight.”The White House stopped short of suggesting a moratorium. Legislatures and governors could also order investigations or a temporary halt to executions. So far, only Oklahoma’s governor has taken action.Missouri’s protocol has been upheld by the courts and Gov. Jay Nixon continues to support “the ultimate punishment” for the “most merciless and violent crimes,” spokeswoman Ansley Channing said.Ohio planned to follow its normal procedures, including an exam of the condemned inmate three weeks before the execution to evaluate his veins and plan for the insertion of intravenous needles.Jerry Cox, president of the National Association of Criminal Defense Lawyers, said the botched Oklahoma execution should “shock the conscience of all Americans,” even those who previously supported the death penalty.“Most of the world and virtually all democracies have abandoned the death penalty,” Cox said. “This is just horrific.”Tuesday’s problems marked the third time this year that an execution raised concerns about an inmate’s suffering.In January, Ohio inmate Dennis McGuire took 26 minutes to die, gasping repeatedly as he lay on a gurney with his mouth opening and closing. That same month, Oklahoma inmate Michael Lee Wilson’s final words were, “I feel my whole body burning.”———Associated Press Mark Sherman in Washington, D.C., contributed to this report.