Two teenagers have stabbed a man to death over an argument about conkers in Edmonton, North London.

Steven Grisales, 21, was about to start a course in architecture. It is reported that Grisales was walking to a railway station when the teenagers started hurling conkers at him. When he asked them to stop, one of the youths took out a knife and stabbed him in the heart.

Two 15-year-old boys are being quizzed by police over the killing, and a 19-year-old man and 16-year-old girl have been arrested and bailed.

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Grisales died the day after the stabbing, which is said to have taken place at 7pm last Wednesday. From Argentina, he had been awarded a scholarship at Westminster College to study architecture, and arrived in London in July.

Flowers and photos have been left as a makeshift memorial to the victim at the site of the crime. Police are looking for three black males in their late teens or early-twenties, and are appealing for witnesses to come forward.

Police are also planning to return to the scene to trace witnesses.

Detective Inspector Richard Beadle, of Scotland Yard's murder command, said: "It happened in a busy residential area and we know there would have been a number of people in the area at the time of the attack. We really need those people to talk to us."