A corrections officer who helped carry out the nation’s first nitrogen gas execution said in a court document that the inmate had normal blood oxygen levels for longer than he expected before the numbers suddenly plummeted.

Another court document indicated that the nitrogen gas was flowing for at least 10 minutes during the execution. The documents filed last month in ongoing litigation provided additional details of the execution of Kenneth Smith, who was the first person put to death using nitrogen gas.

Alabama Attorney General Steve Marshall’s office maintains the high oxygen readings indicate that Smith held his breath as the nitrogen gas flowed, causing the execution to take longer than expected. But attorneys for another inmate said the state has no proof to back up that claim and is trying to “explain away” an execution that went horribly awry.

As the state of Alabama plans additional nitrogen gas executions, questions and disagreements continue over what happened at the first one. A federal judge on Tuesday will hear arguments in a request to block the state from executing Alan Miller by nitrogen gas in September in what would be the nation’s second nitrogen execution.

Media witnesses to Smith’s execution, including The Associated Press, said that Smith shook on the gurney for several minutes before taking a series of gasping breaths. Alabama had assured a federal judge before the execution that the new execution method would quickly cause unconsciousness and death.

A pulse oximeter showed that Smith had oxygen levels of 97% to 98% for a “period of time that was longer than I had expected,” the corrections captain said in a sworn statement. The corrections captain said he did not observe Smith make any violent or convulsive movements, but he did tense up and raise his body off the gurney. After “he released a deep breath,” the oxygen levels began dropping, the corrections captain said.

“The best explanation of the testimony is that Smith held his breath and lost consciousness when he breathed nitrogen gas — not that the mask did not fit or that the nitrogen was impure,” the Alabama attorney general’s office wrote in a court filing.

Attorneys for Miller responded that the state has no evidence to back up that claim and said it would be impossible for someone to hold their breath for as long as the execution took. Instead, they suggested other problems with the mask accounted for the delay.