Alison Steiner with the Office of Capital Defense Counsel told the Mississippi Supreme Court on Monday that the trial judge previously disallowed two other statements as involuntary because they were taken while Keller was on painkillers.
Steiner said Keller was shot in the chest while being taken into custody.
She said the third interrogation — a continuation of the earlier ones and taken some 14 hours later — violated the doctrine of "fruit of the poisonous tree." The doctrine provides that evidence obtained from an illegal, coercive interrogation must be excluded from trial.
Steiner said Keller's physical condition prevented him from signing away his Miranda rights to answer any questions. If authorities had waited three days or more, she said Keller's condition might have been different.
"He was under a lot of medication," Steiner said, and unable to consent to questioning.
Keller was sentenced to die for the 2007 robbery and murder of 41-year-old Hat Nguyen in Harrison County.
Prosecutors said the single mother of four was killed at the convenience store she owned. Nguyen was shot four times.
Keller, now 33, was convicted in Harrison County Circuit Court in 2009.
Assistant Attorney General Jason Davis said the court record showed Keller signed a waiver and agreed to be interviewed by investigators. He said there was no testimony about Keller being impaired by morphine or other drugs.
Davis said Keller responded to questions clearly in the third interrogation and was not incoherent. He said officers did not use information from the previous statements in the third interrogation.
Davis said the trial judge found that Keller's condition was markedly different during the third interrogation from his state of mind in the first two statements. Davis said the judge found Keller was alert and responded coherently to the officers' questions.