A US lawyer who witnessed the execution of a condemned Ohio killer of two says she thinks mistakes were made.
Defence lawyer Carol Wright tried unsuccessfully to leave the witness room to alert a federal judge that there appeared to be problems.
Ms Wright said she believed the rising and falling of Gary Otte's chest indicated he was suffering a phenomenon known as air hunger.
His stomach rose and fell several times, resumed after a consciousness check by guards at 10.42am, then appeared to fall still a couple minutes later.
The time of death was 10.54am local time.
Ms Wright eventually reached the US federal judge overseeing Otte's case, but it was too late.
Otte, 45, had unsuccessfully argued Ohio's lethal-injection method put him at risk of suffering serious pain.
He sang a Christian hymn and quoted the Bible in the minutes before his death.
The last words were derived from a Bible account of Jesus Christ's crucifixion.
He said: "Father, forgive them for they know not what they're doing. Amen."
He earlier professed his love for his family, said he was sorry and sighed deeply, then began singing The Greatest Thing, with such words as "I want to know you Lord".
Relatives of his two 1992 victims, killed in separate robberies, were among the witnesses.