A man who was stabbed to death on a train in Surrey was with his 14-year-old son at the time, police have said.
A manhunt is under way for the 51-year-old victim’s killer, who fled from the train after the stabbing during a “vicious fight” on board the London-bound service.
The victim and the attacker had both boarded the train at the same station.
British Transport Police say they do not believe the pair were known to each other.
Supt Paul Langley said the teenager would have seen the killing.
“This would have been an horrific and hugely traumatic event to have witnessed and we are providing him with as much support as possible,” he said.
Early indications suggested the boy’s father received multiple stab wounds in the attack.
The offender is described as black, in his 20s or 30s, with a slim build and a beard, dressed all in black with white trainers.
People have been urged not to approach him but to call 999.
Police have said the suspect and victim had got on the train at London Road (Guildford) at 13:01 GMT and the offender left the service at Clandon, five minutes later. Officers boarded the train at the next stop, Horsley.
Describing the attack as “violent and deadly”, Supt Langley said there had been a number of sightings of the suspect, who fled towards Dedswell Drive.
He said officers were in the area looking for the killer and BTP was working closely with Surrey Police and the National Police Air Service.
An eyewitness to the killing told the BBC there had been a “vicious fight” on the train.
One social media user who said she had been on the train praised the “incredible” response of the guards and driver in a tweet to South Western Railway.
She wrote: “I was on this train, how those guards and especially that train driver went into action was nothing short of incredible.”
Oliver Thompson, a 16-year-old student living in Clandon, said he was stopped by police in the area at roughly 14:30 and asked if he had seen a “man running from the station”.
A little later he saw police helicopters circling the area before being joined by “a police van with high-powered lights” as night fell.
“It is definitely unnerving to think there is a murderer maybe somewhere in the village.”
Matt Phelps, 35, who works at a business on Epsom Road in West Clandon, said there were three helicopters circling overhead when they locked up and went home.
“To think there was someone like that potentially hiding around the corner somewhere certainly made us want to pack up and go home quicker than usual,” he said.
After the attacker left the train at Clandon, emergency services were called to help the injured man when the train reached Horsley.
Transport Secretary Chris Grayling described the stabbing as “tragic” and said: “Our deepest sympathies are with the victim and their family.”
The train remained at Horsley station for several hours, blocking the line into London Waterloo.
Rail services were suspended between Guildford and Effingham Junction while police carried out investigations, but South Western Railway has said trains are now calling at Clandon and Horsley again.
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