Christopher Vialva. Photo / AP

Warning: Graphic content

The harrowing final moments of a death row killer have been revealed, with the murderer's mum admitting her son looked right at her before he died.

Christopher Andre Vialva, 40, died by lethal injection late last week, more than 20 years after he murdered a young couple in the US state of Texas.

Vialva's death was the seventh execution of a federal inmate in three months, despite pleas from his lawyers that he was not mentally an adult at the time of the crime, being only 19 years old.

Vialva died at 6.46pm at the federal prison in Terre Haute, Indiana last Thursday after his final appeal was rejected by the Supreme Court.

In his final statement, Vialva asked God to comfort the families of Todd and Stacie Bagley, the couple he killed.

The Bagleys, from the US state of Iowa, were robbed at gunpoint as they headed home from church in 1999.

According to court records obtained by the Texas Tribune, Vialva and his co-defendant and fellow death row inmate Brandon Bernard carjacked the couple before locking them in the boot as they attempted to pawn Stacie's wedding ring and pull money from their bank accounts.

Vialva eventually shot both of the victims before Bernard set the car on fire in an attempt to conceal the crime.

Court documents stated the Bagleys sung "Jesus loves us" as the car was doused in lighter fluid and set on fire.

Vialva's mother watched her son's execution from the prison's viewing room.

An Associated Press reporter in the room detailed what Vialva experienced before his death.

The 40-year-old, who was strapped to the table where authorities administer the fatal dose of pentobarbital, reportedly opened his eyes wide as the poison flooded into his body.

After scanning the room, Vialva gave a brief yawn, frowned and then turned his head to look at his mother.

Minutes later, Vialva had stopped moving, his head fixed towards his mother with his mouth open.

An official waited 20 minutes before walking into the death chamber and listening to Vialva's chest with a stethoscope.

Moments later, a voice over the intercom declared Vialva had died at "6.42pm". However, his time of death was later pushed four minutes to 6.46pm. The justice department offered no explanation.

In a YouTube video released earlier this month, Vialva called for a stop to his upcoming execution.

"I'm not making this plea as an innocent man, but I am a changed and redeemed man," Vialva said in the video, released by his lawyer.

Wearing thick-framed glasses, a knitted cap and a prayer shawl over his prison clothes, Vialva admitted he wasn't the "stupid kid" he was in 1999.

"I committed a great wrong when I was a lost kid and took two precious lives from this world… I'm not the stupid kid I was the day I made the most desperate and tragic decision of my life," he said.

President Donald Trump's administration has pushed to restart the federal death penalty this year with authorities executing seven inmates in the past three months.

Trump, who is pushing a "law and order" strategy ahead of the November election, decided to renew federal executions last year.

After various legal developments, the administration succeeded in resuming federal executions in July, and has now put seven to death, including a Native American whose execution was opposed by the Navajo Nation. The other five condemned men were white.

The death penalty is on the decline in the US, where only a handful of states – particularly in the south – still use it. Twenty-two executions took place in 2019 and 14 since the beginning of 2020, including the seven carried out by federal authorities.

Opinion polls show support for the death penalty is on the decline among Americans, though it remains strong among Republicans.

Since the US Supreme Court reinstated the death penalty nationally in 1976, only three men on federal death row were executed before 2020 – the Oklahoma City bomber Timothy McVeigh in 2001 and two Texan murderers in 2001 and 2003.

Vialva was convicted in federal court and not state court because the murders were committed on a secluded part of the Fort Hood US Army post in the Texan town of Killeen.

- With wires