Lockett was declared dead at 7:06 p.m.
Autry, Lockett's attorney, was immediately skeptical of the department's determination that the issue was limited to a problem with Lockett's vein.
"I'm not a medical professional, but Mr. Lockett was not someone who had compromised veins," Autry said. "He was in very good shape. He had large arms and very prominent veins."
The American Civil Liberties Union of Oklahoma, which was not a party in the legal challenge to the state's execution law, called for an immediate moratorium on state executions.
"This evening we saw what happens when we allow the government to act in secret at its most powerful moment and the consequences of trading due process for political posturing," said ACLU executive director Ryan Kiesel.
In Ohio, the January execution of an inmate who made snorting and gasping sounds led to a civil rights lawsuit by his family and calls for a moratorium. The state has stood by the execution but said Monday that it's boosting the dosages of its lethal injection drugs.
A four-time felon, Lockett was convicted of shooting 19-year-old Stephanie Neiman and watching as two accomplices buried her alive in rural Kay County in 1999. Neiman and a friend had interrupted the men as they robbed a home.
Warner had been scheduled to be executed two hours later in the same room and on the same gurney. The 46-year-old was convicted of raping and killing his roommate's 11-month-old daughter in 1997. He has maintained his innocence.
Lockett and Warner had sued the state for refusing to disclose details about the execution drugs, including where Oklahoma obtained them.
The case, filed as a civil matter, placed Oklahoma's two highest courts at odds and prompted calls for the impeachment of state Supreme Court justices after the court last week issued a rare stay of execution. The high court later dissolved its stay and dismissed the inmates' claim that they were entitled to know the source of the drugs.
By then, Fallin had issued a stay of her own — a one-week delay in Lockett's execution that resulted in both men being scheduled to die on the same day.