A man has been arrested over the murder of a pensioner who was stabbed after going to the aid of his neighbour - a convicted paedophile - during a robbery.
After the death of Graham Buck on Saturday, Hertfordshire Police appealed for help in tracing 55-year-old Ian John McLoughlin who they wanted to question in connection with the stabbing.
McLoughlin, also known as Ian John Baker, was arrested in London at around 1am and is being questioned on suspicion of murder and robbery, the force said.
Mr Buck, 66, received fatal stab wounds when he responded to cries from the home of 86-year-old Francis Cory-Wright in the village of Little Gaddesden, near Berkhamsted.
McLouglin 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.
Police said Mr Buck, who lived two doors away from Mr Cory-Wright, went to help Mr Cory-Wright after hearing shouting in his front garden.
Mr Cory-Wright was jailed for 30 months in 2011 for indecently assaulting a 10-year old boy in the 1970s. Mr Buck, a father of three and grandfather, and Mr Cory-Wright are not believed to be connected in any other way than being neighbours.
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.