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Montgomery was sentenced to death after murdering a 23-year-old pregnant woman before cutting out and kidnapping her baby in 2004.
A judge had delayed her execution whilst awaiting a mental health assessment, but the US Supreme Court overturned that decision and the 52-year-old was executed in the early hours of this morning at the Federal Correctional Complex in Terre Haute, Indiana.
A witness to the execution has confirmed that the killer refused to give any final words before her sentence was carried out, and said that she gasped for air as the pentobarbital - the drug administered for a lethal injection - entered her veins through an IV drip.
Montgomery became the first woman to be put to death by the state since 1953 after a lengthy legal battle and a number of delays prevented her sentence being carried out.
Michael Tarm, a legal writer present at the execution, said that Montgomery remained calm and silent before the procedure, and died in silence.
He wrote: "As the execution process began, a female executioner standing over #LisaMontgomery's shoulder, leaned over, gently removed Montgomery's face mask and asked her if she had any last words," Tarm reported.
"'No,' Montgomery responded in a quiet, muffled voice. She said nothing else."
According to TIME, Montgomery looked 'bewildered' at the journalists watching from behind a glass window and 'tapped her fingers nervously' before closing her eyes to die.
Tarm added: "As the lethal injection began, at around 1:18 a.m., Montgomery kept licking her lips and gasped briefly as the pentobarbital entered her body through IVs on both arms.
"A few minutes later, her midsection throbbed for a moment, but quickly stopped."
Following the execution, Montgomery's attorney Kelley Henry took aim at the court for their actions.
She said: "The craven bloodlust of a failed administration was on full display tonight.
"Everyone who participated in the execution of Lisa Montgomery should feel shame. The government stopped at nothing in its zeal to kill this damaged and delusional woman.
"Lisa Montgomery's execution was far from justice."
Montgomery was sentenced to death for the killing of Bobbie Jo Stinnett in 2007. She strangled the Ms Stinnett - who was eight months pregnant - before cutting out her foetus and pretending it was her baby until her arrest.
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