ST. LOUIS - A bungled execution 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 needle, that would suggest human error was to blame rather than an underlying flaw in the execution system.

If the drugs or the secrecy surrounding them played a role, defense attorneys could have a wider legal opening to attack the injection method, plus powerful new evidence to press the U.S. Supreme Court to get involved, legal experts say.

A day after the execution went awry, 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."

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.

Writhing on gurney

Lockett, 38, 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 began 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.

She said she still supported the death penalty and believes Warner should be put to death.

"I believe the legal process worked," Fallin said. "I believe the death penalty is an appropriate response and punishment to those who commit heinous crimes against their fellow men and women. However, I also believe the state needs to be certain of its protocols and procedures for executions and that they work."

'Fell short'

The White House said the execution fell short of the humane standards required.

"We have a fundamental standard in this country that even when the death penalty is justified, it must be carried out humanely," Jay Carney, the White House press secretary, said Wednesday during a briefing. "And I think everyone would recognize that this case fell short of that standard."

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 last 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 G. Roberts Jr. said in the court's 2008 decision upholding Kentucky's lethal-injection system.

In 2011, Hospira, the sole manufacturer of a key lethal-injection drug, sodium thiopental, announced that it would exit the market. That led to a cascade of shortages as states began running out of that and other drugs, in large part because of the European Union's opposition to the death penalty.

With uncertainty about getting the drugs and legal fights, some states have mulled reviving options such as the firing squad (Wyoming), the gas chamber (Missouri), and the electric chair (Virginia).

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.

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.

A minority of the high court has shown some recent trepidation about the secrecy of the process used by many states to acquire the drugs.

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."

Though executions are still continuing in the United States, their numbers have dropped significantly in recent years: Between 1997 and 2005, the nation averaged 71.1 executions each year; between 2006 and 2013, that number dropped to 44.3 executions per year.

This article contains information from the Washington Post.