(Reuters) – A lawyer for an executed Arkansas death row inmate asked the state on Friday to investigate why his client coughed and convulsed on a death chamber gurney, saying a lethal injection drug may have been the cause.

A separate group of lawyers for Arkansas death row inmates asked a federal court to preserve evidence in the four executions Arkansas held over eight days this month, saying in a lawsuit that the state’s protocols “did not prevent an execution by torture.”

Arkansas, which had not held an execution in 12 years, concluded its executions series by putting to death Kenneth Williams on Thursday night. Accounts of his execution raised fresh concerns about whether the sedative midazolam, a Valium-like drug, is effective in lethal injection mixes.

Witnesses said Williams, who admitted to killing four people, jerked and gasped for air for about 30 seconds a few minutes after his execution began. The state said it was a routine execution lasting about 15 minutes, but critics said something was amiss.

“It is not a normal reaction to therapeutic doses of midazolam,” said Jonathan Groner, a professor of surgery at the Ohio State University College of Medicine who has testified against the drug’s use in executions.

“Was the drug doing what the state intended it to do or was the person being chemically waterboarded on the way to being killed?” he asked in an interview when talking about execution mixes.

Shawn Nolan, a lawyer for Williams, on Friday asked Arkansas to investigate his execution.

The lawsuit filed in a U.S. district court in Little Rock said: “If the midazolam fails to keep the prisoner under anesthesia, the prisoner would be awake and aware but unable to move or speak or even open his eyes, so he would then look completely serene despite being in agony.”

Republican Governor Asa Hutchinson, who set the hurried execution schedule because the state’s supply of midazolam expires at the end of April, told reporters there was no need for an investigation and all the executions were carried out within Arkansas’ protocols.

The United Nations’ human rights office voiced deep concern on Friday, saying the state’s rush to carry out the executions before a drug expired added to the “arbitrariness and cruelty” of the process.

Midazolam is supposed to render inmates unconscious but critics say it has failed in some cases, leaving them to feel the effects of a paralytic that halts breathing and another drug that stops the heart while causing an excruciating burning sensation.

Major pharmaceutical companies began a sales ban on lethal injections drugs about six years ago to death penalty prison systems due to ethical concerns.

Several states then turned to new mixes that included midazolam. The drug was used in troubled executions in Oklahoma and Arizona where witnesses said inmates twisted in pain on death chamber gurneys.

Death penalty supporters have said some pain in executions is warranted given the brutality of the murders the condemned typically commit and the harm they have inflicted on victim’s families.