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In America more than 2,500 men and women are being held on death row, waiting to be executed.

The US is one of 58 countries across the world to impose the death penalty for the most serious crimes.

Last year, 22 people were lawfully killed in the US, among them murderers and serial killers.

Executions take place in 29 states with Texas carrying out the most.

The most common way to execute prisoners in the US is now lethal injection - although electrocution, firing squad, hanging and even cyanide gas have been used to kill prisoners.

(Image: Getty Images)

When a death row inmate is executed by lethal injection it is supposed to be a quick and painless death.

The prisoner is injected with at least one, sometimes more, drugs which make them unconscious before stopping their breathing and finally their heart.

Sodium thiopental induces anaesthesia, the the inmate is injected with pancuronium bromide to relax muscles and finally they are given potassium chloride to stop the heart.

But increasingly questions are being asked as to how humane the method of execution really is.

More than 1,000 people have now had their lives ended by lethal injection in the US and of those, 75 of them were "botched".

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Serial killer Stephen Peter Morin, who targeted women to murder them.

He had a history of drug abuse and when he was put to death in 1985, executioners had to spend 45 minuts prodding his arms and legs looking for a vein before they could find a suitable one.

There are several cases where it took more than an hour for executioners to find a vein they could use on death row prisoners.

One, Randy Woolls, even helped those who were putting him to death find a useable vein.

In one gruesome execution, murdered and rapist Stephen McCoy had an extreme reaction to the drugs used in the lethal injection.

(Image: Getty Images)

His chest started to heave, he gasped, choked and then arched his back from the guerney as the execution was taking place.

McCoy's gruesome reaction was so severe one of the male witnesses fainted.

Charles Walker had murdered two people - but when he was executed, he died in unbelievable agony.

There was a kink in the tube that was delivering the deadly drugs to kill Walker, which dragged out his execution.

The needle was also pointing away from his heart, which made Walker's death even more prolonged.

But even when the lethal injection all seems to go the plan, experts are now warning they are far from the painless and humane experience they have been claimed to be.

And there are never any living witnesses to reveal exactly how it feels.

Researchers in Florida and Virginia, which both have the death penalty and use lethal injection, have exposed the distrubing reality of how it feels to die by the deadly drugs.

They found that in 90 per cent of cases prisoners were still conscious when they died - not unconscious as they should be.

(Image: Getty Images)

Horrifyingly, 40 per cent of death row prisoners can feel pain as they are executed.

Leonidas Koniaris, a surgeon from Miami who helped compile the research paper, said: "My impression is that lethal injection as practiced in the US now is no more humane than the gas chamber or electrocution, which have both been deemed inhumane."

Doctors, nurses and medical professionals are banned by their code of ethics from helping with executions, even to ease the pain of the person being put to death.

This means the technicians who administer the anaesthetic to ensure the prisoner feels no pain have no specific medical training.

The harrowing truth of what happens if a death row inmate can feel what is happening it truly harrowing.

They would have been aware of the asphyxiation, then they would have felt as if they were being burned from the inside out.

Finally, they would suffer massive muscle cramping before feeling their own final heartbeats.

Even more horrifyingly, because of the huge amounts of muscle relaxant used on the prisoners means they are utterly unable to report the immense pain they are feeling.

In the US, vets are banned from using muscle relaxants when they are euthanising pets so they can make sure the lethal injection is working and the animal is not in any pain.

Mr Koniaris said: "When you look at it critically, it’s anything but medical. “It’s a perverted medical practice."