A "totally selfless" man was murdered after going to the aid of his neighbour - a convicted paedophile - during a robbery, police have said.
Graham Buck, 66, received fatal stab wounds on Saturday afternoon when he responded to cries from the home of 86-year-old Francis Cory-Wright in the village of Little Gaddesden, near Berkhamsted, Hertfordshire police said.
Police said they were seeking to question 55-year-old Ian John McLoughlin, also known as Ian John Baker, in connection with the murder.
They have warned the public not to approach McLoughlin and have described him as "extremely dangerous".
Detective Chief Inspector Martin Brunning, of Bedfordshire, Cambridgeshire and Hertfordshire major crime unit, told a news conference: "The picture emerging is that Mr Buck, who lives two doors away from Mr Cory-Wright, became aware of shouting from the premises, as we believe by Mr Cory-Wright, and went to his assistance based on what he heard from his front garden."
Mr Cory-Wright was jailed for 30 months in 2011 for indecently assaulting a 10-year old boy in the 1970s. Police have said Mr Buck, a father-of-three and grandfather, and Mr Cory-Wright were neighbours and are not believed to be connected in any other way.
Detective Chief Superintendent Jeff Hill said Mr Buck, described as a retired financial industry worker, paid the "ultimate price" for intervening during the robbery at the home of an elderly neighbour.
"Mr Buck's actions were totally selfless and illustrate a deep sense of community spirit which deserves recognition and respect," he said.
"His family are receiving support from specialist officers and we extend our deepest sympathies to them at this truly dreadful time."
Police said McLoughlin had no real ties to Hertfordshire and had come to Little Gaddesden on Saturday for a "purely financial motive".
He was on day release from Springhill prison in Aylesbury, Buckinghamshire, after being convicted in 1992 and sentenced to life imprisonment for the murder of 56-year-old Peter Halls, from Brighton.
McLoughlin was 22 years into a 25-year sentence, according to police, and was serving his sentence at Springhill.
He was also jailed for manslaughter in 1983 following the death of 49-year-old Len Delgatty, from Stoke Newington, north London. He was sentenced to 10 years for this offence which was later reduced to eight years on appeal.
Mr Brunning said McLoughlin and Mr Cory-Wright, who was released from jail recently, knew each other "through the prison environnment".