After years of controversy over lethal injection failures, US state now plans to execute inmates with nitrogen gas.

Oklahoma, a US state with one of the busiest death chambers in the country over the past three decades, will have at least a two-year delay in lethal injections after officials declined to consider new execution procedures.

The state’s Board of Corrections did not take up new execution protocols at a regular meeting that Attorney General Scott Pruitt wants in place before executions can resume.

After a botched execution in 2014 and drug mix-ups during the last two scheduled lethal injections in 2015, Pruitt said he wouldn’t request any execution dates for five months after the new protocols are approved and he’s confident the death penalty can be carried out without any problems.

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“I want to assure the victims’ families who continue to await justice that this review process will continue to be both deliberate and empirical,” Pruitt said in a statement.

“The protocol is a work in progress,” said board spokesman Alex Gerszewski. “There currently is no timeline on when anything will be released.”

Meanwhile, five Oklahoma death row inmates have exhausted all of their appeals and are awaiting execution dates.

Oklahoma has executed 112 people since the death penalty was reinstated in 1976, the highest per-capita rate in the nation and second overall only to Texas, where 537 inmates have been put to death over the past 40 years, according to the Death Penalty Information Center.

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Oklahoma was the first American state to authorise lethal injection as a method of execution, and capital punishment has strong, bipartisan support in the Oklahoma legislature.

After the botched execution of Clayton Lockett in 2014 – who writhed on the gurney during a lethal injection that prison officials tried unsuccessfully to halt – politicians approved the use of nitrogen gas as an alternative to lethal injection.

The legislature also passed a resolution seeking a public vote on whether to enshrine the death penalty in the state constitution, making it more difficult for future legislators or the courts to end it. That state question will appear on the ballot in November.

In September 2015 the state determined that doctors carrying out executions there are not obliged to minimise suffering as they are not acting in the capacity of a physician.

Representative Mike Christian, a former policeman and staunch advocate for the death penalty who sponsored both measures, said he’s disappointed prison officials have taken so long to develop new procedures and wants them to include the use of nitrogen gas as an alternative to lethal injection.

Nitrogen gas has never been used to execute inmates in the US.

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“My question is why they don’t at least have some kind of protocols established for nitrogen hypoxia,” said Christian. “It’s second in line behind lethal injection, and I think lethal injection is on its way out the door.”

Still, death penalty opponents voiced concern that Oklahoma appears to be moving in the opposite direction of other states, where executions are being reconsidered altogether.

“The last thing the state of Oklahoma needs to do is rush back into the business of executing people,” said Ryan Kiesel, executive director of the American Civil Liberties Union of Oklahoma. “We would prefer that the state get out of the business of executing its citizens altogether.”