Strapped down with a mask put over their head: BBC viewers watched Silent Witness killers try to murder a victim with gas - just like the Alabama execution of Kenneth Eugene Smith

BBC viewers who tuned in to an episode of Silent Witness saw the show’s killers attempt to murder their victims using a lethal gas in a scene similar to the execution of Kenneth Eugene Smith.

The plot of the latest two part of episode of the long-running crime drama sees a university professor accused of murdering some of his ‘woke’ critics after claiming he was passed over for a promotion for his ‘triggering’ comments.

But in a dramatic twist it emerges that two of the professor’s acolytes are behind the murders and later kidnap two other students who they believe are trying to get him fired from the university.

The killers then tie them both to chairs before placing a gas mask, which is attached to a supply of nitrous oxide, around one of the women’s heads in order to suffocate her to death.

He then turns on the valve, releasing the gas, before putting another mask over the second woman. But seconds later the show’s protagonist Dr Nikki Alexander and her colleague Jack Hodgson burst into the room, putting an end to their evil plot.

The scene draws shocking parallels to the real life execution of convicted killer Kenneth Eugene Smith, who was put to death last night using nitrogen hypoxia for the first time in history.

BBC viewers have tuned in to an episode of Silent Witness  killers attempt to murder their victims using nitrous oxide gas

The killers kidnap two of their victims before tying them both to chairs and placing a gas mask around one of their heads in order to suffocate her to death

But seconds later the show’s protagonist Dr Nikki Alexander and her colleague Jack Hodgson burst into the room, putting an end to their evil plot

The 58-year-old had a firefighters-style mask placed over his face as he was strapped to a gurney inside an execution chamber at the William C. Holman prison in Atmore, Alabama.

Smith was 22-years-old when he was first arrested for the 1988 murder of Elizabeth Sennett.

He was sentenced to death in 1996 after admitting the murder-for-hire killing of the pastor’s wife who was beaten and stabbed.

Experts had said he would be unconscious within seconds of inhaling the gas – but it took up to seven minutes for him to pass out and another 15 for him to be declared dead.

After the gas began to flow into his mask at 7.53pm, the inmate smiled, nodded toward his family and signed ‘I love you’.

Smith made a heart sign with his left hand to his wife Deanna before declaring: ‘Tonight Alabama causes humanity to take a step backwards… I’m leaving with love, peace and light.’

But soon Smith was wrenching against the straps, gasping, panting loudly and also trying to hold his breath to try to avoid inhaling the nitrogen through a mask covering his entire face.

No doctor was present in the chamber but one arrived at the end to declare him dead at a 8.16pm – just over 22 minutes after the execution began.

Before the medic came in to check the body, one correction officer had to repeatedly peep into the mask to see his face and try to ascertain if he still looked alive.

One of the university students has the mask placed over her head during the terrifying scene

The killers plan to suffocate her to death using nitrous oxide gas

One of the killers then turns the valve on the cannister of nitrous oxide which is connected to the gas mask

But before he is able to carry out his evil plot, the protagonists of the BBC drama burst onto the scene and rescue the women

Kenneth Eugene Smith was sentenced to death in 1996 after admitting the murder-for-hire killing of a pastor’s wife who was beaten and stabbed in 1988

Alabama’s lethal injection chamber at Holman Correctional Facility in Atmore, Alabama, is pictured in this Oct. 7, 2002 file photo, where Smith was executed on Thursday evening

Smith’s wife Deanna cried out for her killer husband as he thrashed against his restraints, before breaking down at a press conference following the controversial execution (pictured)

His bereft wife Deanna cried out for her killer husband as he thrashed against his restraints and tried to fight the gas by not breathing it in.

At one point as the killer tore against his restraints, his wife, wearing a T-shirt reading ‘Never Alone’, desperately cried out for him as she sobbed from the witness box.

Next to her was journalist Lee Hedgepeth. He has given the most detailed account of what unfolded at William C. Holman prison.

He told the BBC: ‘The curtains of the execution chamber pulled back. Kenny is laying strapped to a gurney. His upper body is strapped.

‘He is wearing a gas mask that covers his face from his forehead to his chin. The gas mask is strapped in multiple places to his head as well as the gurney.

‘The execution starts at about 7.53. The nitrogen begins flowing into the gas mask and Kenny begins to violently shake against the straps that are holding him.

‘That violent shaking lasts for about four or five minutes. I’ve witnessed five executions in Alabama. Four of them lethal injection. One with nitrogen and this was the most violent execution I have ever seen.

‘Sitting beside me was Deanna Smith, the wife of Kenny. Once Kenny began to violently shake against those straps she began to sob. There were other media witnesses in the room, folks who had seen other executions, and I think there was a shock in the room at how violent the execution was.

‘Doctors are not in the room during the execution. A doctor did pronounce Kenny dead afterwards but no doctor is visibly present in the execution chamber.

‘The execution began around 7.53 and he was declared dead at 8.16 so around 20 something minutes’.

Smith was 22-years-old (seen in his original 1989 mugshot) when he was first arrested for the 1988 murder of Elizabeth Sennett. His conviction was overturned before he was sentenced to death for the same murder at a separate trial in 1996

Elizabeth Sennett, 45, (right) was found dead on March 18, 1988, in the her home in Alabama’s Colbert County. She had been stabbed eight times in the chest and once on each side of neck

Smith’s pastor John Ewell told DailyMail.com before his execution that the killer was ‘really struggling’ with the reality of his imminent death

Jeff Hood, Smith’s spiritual advisor before his execution, described the untested execution method as a ‘horror show’, and said ‘unbelievable evil was unleashed tonight in Alabama’

When asked if more states will use the method to execute inmates he said: ‘We have never seen a execution by nitrogen before. People will have to consider if this is a more humane execution but people in the room believe it was more violent than lethal injection’.

Smith’s pastor John Ewell told DailyMail.com that the killer was ‘really struggling’ with the reality of his imminent death. He barely touched his final meal of a T-bone steak, hashbrowns, eggs and A1 Sauce from Waffle House.

The murderer’s friend and spiritual guide, Jeff Hood, described his death as the ‘worst thing’ he had ever seen, claiming jail officials gasped in shock because he died far slower than anticipated. One prison officer was ‘tap dancing’ with nervousness and others were ‘very surprised that this didn’t go smoothly’, he said.

‘When they turned the nitrogen on, he [Smith] began to convulse, he popped up on the gurney over and over again, he shook the whole gurney,’ Hood said afterwards.

‘I could see the corrections officers, I think they were very surprised that this didn’t go smoothly – one of the state officials in the room was so nervous she was tap dancing,’ he continued.

‘(Smith) kept breathing for what could possibly be up to nine minutes, ten minutes, unbelievable evil was unleashed tonight in Alabama… that was torture.’