Dr Matthew Puncher, pictured, killed himself with self-inflicted stab wounds, an inquest has ruled, although the possibility of someone else being involved in his death was 'not excluded'

A radiation expert who investigated the 'assassination' of Alexander Litvinenko was found dead five months after a trip to Russia, an inquest heard.

Matthew Puncher, 46, bled to death at his home from multiple stab wounds inflicted by two knives.

A pathologist said he could not ' exclude' the possibility that someone else was involved in the death - but concluded the injuries were self-inflicted.

It led a coroner to record that Dr Puncher - who discovered the amount of toxic polonium inside ex-KGB agent Litvinenko after he drank poisoned tea in London in 2006 - committed suicide.

The inquest also heard the father-of-three had 'become obsessed' with a coding error he made in his research which he feared could 'land him in prison' for breaking a contract with the US Government, while his wife said his mood had 'completely changed' following a trip to Russia.

Dr Puncher was found dead at his home in Drayton, Oxfordshire, in May with stab wounds to his arms, neck and upper abdomen.

He was an expert in radiation protection dosimetry and worked for Public Health England at the UK's Atomic Energy Research Establishment at Harwell, Oxfordshire.

He had been given sole responsibility over a contract with the US Federal Government for a programme measuring polonium levels inside people who previously worked on the USSR's nuclear weapons.

Head of department at Harwell, George Etherington, described Dr Puncher's concerns that his 'miscalculation' of the effects of the radiation on the workers would land him prison as 'irrational'.

The inquest also heard that redundancies and restructuring at Public Health England's premises in Harwell, near Didcot, had resulted in Mr Puncher receiving a much greater workload.

Colleague George Etherington said in a statement that Dr Puncher was a senior scientist working in the area of radiation exposure.

He said: 'About three to five years ago we began to carry out contractual work for the American Federal Government working on risks of exposure to plutonium.

'About two to three years ago Matt was asked to take the lead and in April 2015 the restructuring was completed,' he said.

Dr Matthew Puncher, who investigated the 'assassination' of ex-KGB agent Alexander Litvinenko, pictured, committed suicide, an inquest ruled

'In February 2016 he attended a meeting in Russia and when he returned he appeared to be quiet and more confined and people told me mistakes made in mathematical analysis came to light during this meeting.

'On April 27 I could see Matt was stressed and he said he had made a mistake and he thought he could be prosecuted for not meeting the contractual commitment.

'He said it was having an effect on his marriage but said his wife was very supportive but he had been having thoughts of suicide. I told him his fears were groundless and he would look back and wonder why he worried so much.'

The inquest heard Dr Puncher had told other workers about the error in his paper which was rectified in an addendum but that was something he believed was a way of 'fudging it.'

He also told colleagues he thought he was having a nervous breakdown and his brain was not working. It was also revealed during the inquest that his work had been evaluated as excellent and he was in line for a bonus.

However the bonus was blocked by the Human Resources department due to only a certain amount of employees being entitled to the bonus payments. He also raised concerns about the travel conditions for his journey to Russia.

Detective Constable Rachel Clarke, who investigated the death, said: 'His injuries were so extensive, I didn't know how he could have inflicted them on himself without becoming unconscious so we looked at the wider circumstances.'

DC Clarke said there was no evidence of a disturbance or a struggle and no evidence of anyone else's blood.

Dr Puncher worked at the UK's Atomic Energy Research Establishment at Harwell, Oxfordshire, ringed, but the inquest heard he feared a 'coding error' he made could land him 'in prison'

She added: 'It was very unusual. All the information told us he was very depressed and no-one in his family seemed particularly surprised he had taken his own life.'

His post mortem was carried out by Home Office pathologist Dr Nicholas Hunt, who said it was possible for a person to self-inflict the number of wounds he did before becoming unconscious

Dr Hunt also conducted the post mortem on weapons expert Dr David Kelly in 2003, who was found dead in Oxfordshire woodland around 10 miles from where Dr Puncher lived.

The Hutton Inquiry concluded that Dr Kelly had committed suicide.

Dr Puncher's wife Kathryn, who was married to him for 16 years, told Oxford Coroner's Court his mood 'completely changed' after the visit to Russia before Christmas.

She said: 'He was always an upbeat and sensitive man.

'He was brilliant with the children as he was so intelligent and enjoyed helping them with their homework.

BATTLE FOR JUSTICE: THE DEATH OF ALEXANDER LITVINENKO A public inquiry concluded earlier this year that the killing of Mr Litvinenko - an outspoken critic of Vladimir Putin - who died in 2006 after drinking tea laced with radioactive polonium-210 - had 'probably' been carried out with the approval of the Russian president. The inquiry headed by the former high court judge Sir Robert Owen found two Russian men - Andrei Lugovoi and Dmitri Kovtun - had deliberately poisoned Mr Litvinenko by putting polonium-210 into his drink at a London hotel, leading to an agonising death. It said the use of the radioactive substance - which could only have come from a nuclear reactor - was a 'strong indicator' of state involvement and that the two men had probably been acting under the direction of the FSB. A UK public inquiry found Andrei Lugovoi, pictured, was one of two men who 'deliberately poisoned' Mr Litvinenko by putting polonium in his drink at a London hotel Possible motives included Mr Litvinenko's work for British intelligence agencies, his criticism of the FSB, and his association with other Russian dissidents, while it said there was also a 'personal dimension' to the antagonism between him and Mr Putin. International arrest warrants issued for Mr Lugovoi and Mr Kovtun remain in force although Russia continues to refuse their extradition. In a statement to mark the 10th anniversary of his death, Marina Litvinenko said her husband - who she called Sasha - had been an 'extraordinary man' whose courage in speaking out against the Russian security service, the FSB, had left an enduring legacy. While she acknowledged Mr Putin had refused to accept the inquiry's findings, she said it remained open for other world leaders to take action against the Russian state and that she hoped her struggle to find the truth had not been in vain. Dmitry Kovtun, pictured, was also found responsible for the death by the inquiry 'It has taken 10 long years for the truth to be established and for Sasha's dying words that President Putin was responsible for his death to be proved to be true,' she said. 'I know that Mr Putin's Russia does not accept the findings of the British public inquiry and will continue to deny the truth in the face of overwhelming evidence. 'But those findings are now part of history and the rest of the world understands the difference between truth and propaganda. And that is what matters to me. 'What action world leaders will take against the ever vengeful Russian state in these dramatic times remains to be seen. I hope and pray that my struggle has not been in vain.' Advertisement

'After Christmas he changed completely. He just lost interest and I had to prompt him to do things like getting dressed and washing up, things he did without thinking before.

'He used to cook all the meals but he just stopped. He seemed to stop caring about anything.'

Dr Puncher and a colleague calculated the amount of polonimum-210 found inside Alexander Litvinenko's body following his death in 2006.

Litvinenko was a former officer of the Russian FSB secret service who specialised in tackling organised crime.

He fled with his family to London and was granted asylum in 2000 and worked as a journalist, writer and consultant for the British intelligence services.

In January 2016 a UK public inquiry concluded the defector was probably murdered on the direct orders of the Russian security service FSB, approved by President Putin.

Oxfordshire Coroner Nicholas Graham recorded a verdict of suicide.