A convicted killer with an IQ of 140 is being hunted by police after a good samaritan pensioner was violently killed in a quiet Home Counties village.
Police said Ian McLoughlin was a violent man who may have fled abroad and should not be approached.
It comes after Graham Buck, 66, was stabbed to death on Saturday afternoon when he responded to cries from the home of 87-year-old Francis Cory-Wright in Little Gaddesden, near Berkhamsted in Hertfordshire.
McLoughlin, who was on day release, had heard of Cory-Wright, a convicted paedophile, in prison, police said.
Detective Chief Inspector Martin Brunning said that McLoughlin had arrived at the house and for "some considerable time" there was a "cordial conversation going on."
"It would appear that the meeting was cordial, certainly to start with," Brunning said.
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.
Hill said Mr Cory-Wright was injured during the incident, but has since been discharged from hospital. He said he is now in a "place of safety".
McLoughlin 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.
McLoughlin, 55, is described in The Times as a 'bisexual who hates homosexuals', with both men he has previously killed having been gay.
He grew up in Cumbria, and was repeatedly in trouble with the police as a child, it was reported.
Hill said: "Clearly McLoughlin is a violent man who needs to be apprehended.
"We would advise that people continue with their daily routines but be vigilant and report any potential sightings immediately."
"However Ian McLoughlin may well have left the county - the hunt has gone nationwide with ports authorities, police forces, airports across the country all being notified and put on alert," he said.
He said police were particularly appealing to guest house owners, hoteliers and people working in transport and pubs to be "vigilant". He said McLoughlin was known to drink heavily.
"This man should not be approached if seen and any sighting should be reported to the police via 999 immediately," he said.