The Supreme Court is stepping into the issue of lethal injection executions for the first time since 2008 in an appeal filed by death row inmates in Oklahoma.

The justices agreed Friday to review whether the sedative midazolam can be used in executions because of concerns that it does not produce a deep, comalike unconsciousness and ensure that a prisoner does not experience intense and needless pain when other drugs are injected to kill him. The order came eight days after the court refused to halt the execution of an Oklahoma man that employed the same combination of drugs.

Oklahoma, as well as Florida, uses midazolam as one of three drugs in lethal injection executions. The second drug serves to paralyze the inmate and the third one is used to stop his heart.

The case will be argued in late April, an attorney for the men said Friday. A decision is expected by the end of June.

The appeal was brought to the court by four Oklahoma inmates with execution dates ranging from January to March. The justices allowed Charles Warner to be put to death on January 15 and denied stays of execution for the other three.

At the time, Justice Sonia Sotomayor wrote a dissent that was joined by three other justices, calling on the court to examine whether the drug could be used in accordance with the constitutional prohibition on cruel and unusual punishment.

Friday's order does not formally call a halt to those scheduled procedures. Dale Baich, an attorney for the inmates, said he would ask the court to block the executions until the case is decided.

"Oklahoma's execution protocol has been affirmed as constitutional by two federal courts," Oklahoma Attorney General Scott Pruitt said Friday. "We will continue to defend the constitutionality of this protocol in order to preserve (the Department of Corrections') ability to proceed with the sentences that were given to each inmate by a jury of their peers."

In 2008, the justices upheld the use of a different three-drug combination in a case from Kentucky and set a high bar for challenges to lethal injections. Chief Justice John Roberts wrote then that the court probably would not stop executions unless "the condemned prisoner establishes that the state's lethal injection protocol creates a demonstrated risk of severe pain."

What has changed since 2008 is that states have been forced to change the drugs they use in executions after drug manufacturers took steps to ensure their products are not used in executions.

The inmates are trying to stop their executions, arguing that the state would essentially be experimenting on them by injecting them with unproven and untested drugs.

"The drug protocol in Oklahoma is not capable of producing a humane execution, even if it is administered properly," Baich said.

Last April, Oklahoma used midazolam for the first time in a grisly procedure. Inmate Clayton Lockett clenched his teeth, moaned and writhed on the gurney before a doctor noticed a problem with the intravenous line and the execution was called off. Lockett died 43 minutes after the procedure began.

Oklahoma revamped its procedures in response to the Lockett execution, including a fivefold increase in the amount of midazolam used. In last week's execution, Warner showed no signs of physical distress.

Florida used the same procedure in an execution carried out the same night and has scheduled the execution of Jerry Correll for Feb. 26.

Arizona and Ohio, which had problem-filled executions involving midazolam and a second drug, have said they won't use that drug mixture again.

The unusual turn of events in which the court allowed an execution to proceed then decided to hear an appeal initially filed by the dead man and three other inmates can be partly explained by the court's internal practices. The votes of four justices on the nine-member court are enough to grant an appeal. But it takes a majority of five justices to block an execution.

An informal and inconsistent practice has in the past provided a "courtesy fifth" vote in situations similar to the one in Oklahoma. It is unclear why no justice was willing to do that last week.

Joining Sotomayor were Justices Ruth Bader Ginsburg, Stephen Breyer and Elena Kagan.

Justices Samuel Alito, Anthony Kennedy, Antonin Scalia, Clarence Thomas and Roberts voted to allow the execution to go forward.