Two people have been arrested after a man was stabbed to death on a train in Surrey in front of his 14-year-old son.
The 51-year-old victim was pronounced dead in the carriage at Horsley station after suffering multiple stab wounds in a "violent killing" on a train travelling between Guildford and London Waterloo.
Officers attended an address in the Farnham area at about 6am and detained a man on suspicion of murder.
A 27-year-old woman was also arrested on suspicion of assisting an offender.
Police say they do not believe the attack was random.
British Transport Police Assisant Chief Constable Sean O'Callaghan told Sky News the men joined the train together at Guildford and a discussion occurred between them on the train.
"That continued for between three and four minutes," he said. "During the course of that discussion, both men moved to a second carriage, and that is where the violence ensued to the veracity that we are now aware of and tragically that is where the man lost his life."
DCI Sam Blackburn, also from British Transport Police, said: "The investigation is moving with good progress and we are now confident to say that this is not believed to be a random assault. In the moments leading to the violent killing, both men appeared to be involved in an altercation lasting three minutes.
"Nothing justified the extraordinary violence that followed, and we are concentrating our efforts on the ongoing investigation."
The attack happened on the 12.58pm service travelling from Guildford to Waterloo on Friday. Officers were called to Horsley station at 1.15pm.
The victim's family, including his teenage son who was nearby at the time of the attack, are being supported by police officers, and a post-mortem is scheduled for early next week.
DCI Blackburn added: "We are continuing our appeal for information and I would urge anyone who was on board the train to contact us as soon as possible."
Superintendent Paul Langley said the attack would have been a "horrific and hugely traumatic event to have witnessed".
Police have described the suspect as a black man in his 20s or 30s, of slim build with a beard and around 6ft tall, dressed all in black with white trainers.
A woman who lives locally reportedly warned other villagers to be aware after speaking to a "very sweaty and nervous man" in a field near her home.
Police said the knifeman left the train at Clandon station and headed in the direction of Dedswell Drive.
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Transport Secretary Chris Grayling tweeted: "The incident near Horsley is tragic, and our deepest sympathies are with the victim and their family. I urge anyone who witnessed what happened to contact @BTP and @SurreyPolice, by texting 61016 or calling 0800 40 50 40."
The train operator said all lines had now been reopened with services returning to normal.
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