Image caption Dr Richard Shepherd has conducted post-mortems for more than three decades

Forensic pathologist Dr Richard Shepherd has examined evidence in some of the most high-profile and tragic deaths in recent history - the 9/11 attacks, the 7/7 bombings, the deaths of Princess Diana and Stephen Lawrence. It has taken a mental toll.

"Two hundred damaged and dismembered bodies in one place leaves an image," Dr Richard Shepherd tells the BBC's Victoria Derbyshire programme, explaining how his career as a forensic pathologist has had deep and traumatic effects on his mental health.

"I am used to death, and have been used to it for 35 years - but there comes a moment when you can't compartmentalise it."

Dr Shepherd estimates he has conducted more than 23,000 post-mortem examinations in his career, many from mass killings in some of the greatest tragedies of the past few decades.

It has left him with Post-Traumatic Stress Disorder (PTSD).

"Maybe I should have gone to see a counsellor once a year," he reflects, adding that he did not feel the need until the condition developed in his sixties, the prime of his career.

The trigger had been ice cubes in his drink - reminding him of his work after the Bali bombings of 2002, when there was no refrigeration for the piles of corpses.

But he believes the roots may have been set more than a decade earlier.

'Bizarre and disturbing'

"In the plane over Hungerford was the first indication," he says - referring to the massacre in 1987 in which 16 people were killed by gunman Michael Ryan.

It had been Dr Shepherd's first major case.

"There was a resonance which was bizarre and disturbing, and it built from there."

At times, he says in his new book, he feared closing his eyes as his mind would be "pursued by body fragments.

"There were intestines. Spongy livers. Hearts that did not beat. Hands. The clawing stench of decay that took my breath away."

At times, he adds, "I thought it better to die than to live like this".

It is this personal experience that has led him to believe that after mass-disasters, pathologists should be given a psychological debrief, as are other emergency services personnel.

Most forensic pathologists are self-employed, so they miss out, he tells the BBC.

Image copyright Metropolitan Police Image caption Dr Shepherd conducted post-mortems following the 7/7 terror attacks

He wants to make it clear, however, that post-mortem examinations are not a brutal act.

"I can understand the perception, but it's actually complex surgery and doesn't make the body look horrendous.

"After 7/7 [the 2005 London attacks], the reconstructions allowed many of the relatives to have a final moment and say goodbye.

"Nobody after a post-mortem looks worse than when they came in," he says, adding that the closure of knowing what happened to the deceased can help relatives try to rebuild their lives.

'Truth is crucial'

At the heart of all his work, he says, is to find the truth of what happened.

And it is this that underpins the work of which he is most proud - including his investigation into the death of a 15-year-old girl who died of sudden epilepsy death, or Sudep.

"I hope I gave the parents a true understanding of what happened to their daughter," he says.

"I'm a strong believer truth is crucial.

"If they have the whole truth it's fine - if it's not the whole truth they will discover a gap and not believe anything you say."

The most common question he is asked by relatives of the deceased is whether they felt any pain.

"I just give relatives the truth," he says.

While he tries to be gentle with his words, he says he "can't make it better or worse.

"If they have asked the question, it's up to me to answer truthfully."

Image caption When investigating the murders of Harold Shipman (above), Dr Shepherd says he had a "real mental struggle" believing a GP could be a serial killer

It is clear that Dr Shepherd cares about each individual case he has taken up.

"It's someone's family member," is what he keeps in mind with each assessment.

"Each case is a unique puzzle and their death is unique."

This was especially true of some of his most high-profile cases.

He admits he had a "real mental struggle" believing that someone of Harold Shipman's position - a trusted GP responsible for the deaths of up to 215 patients - could commit such horrific acts.

"But the evidence all pointed that way" he says. "And it was the truth."

He was also involved for nearly a decade in the investigation into the death of Stephen Lawrence, the 18-year-old who was stabbed to death in a racially motivated attack in Eltham, south London, in 1993.

An investigation later found there had been "institutional racism" in the police.

Dr Shepherd says "pathologically, it was very straightforward" even though "everything around it was so complicated".

"The two stab wounds could so easily have been non-lethal. It was so sad," he adds.

Testing theories on dinner meat

Dr Shepherd has now retired from work with the Home Office - but he still works on referral cases for the defence in court cases.

His specialism is in knife-related incidents. He says his experience means that in some cases, with multiple stabbings, it has been possible from the wounds to draw an accurate picture of what the knife would have looked like.

But his work has also seeped into his home life - including using the meat served at dinner as a tool for trying out his theories with knives of differing lengths and at different angles.

"I didn't stab every Sunday roast with a variety of knives, but the children definitely did see me do it sometimes," he admits.

"How else do you assess these things? Why waste an opportunity?"

Watch the BBC's Victoria Derbyshire programme on weekdays between 09:00 and 11:00 BST on BBC Two and the BBC News Channel in the UK and on iPlayer afterwards.