Four people have been found guilty of the murder of an NHS worker.

David Gomoh, 24, was repeatedly stabbed in the street on 26 April last year as he spoke to his girlfriend on the phone. The marketing graduate, who worked in NHS procurement, staggered back to his home in east London and bled to death in front of his horrified family.

Jurors heard the victim had no connection with gangs, but was nonetheless targeted in an ongoing feud involving the “petty but fatal game of one-upmanship” played out on the streets of London.

Afterwards, investigators uncovered sketches by one of his killers that revealed a chilling “picture-book story” of the events leading up to the murder.

David Ture, 19, of no fixed address; Vagnei Colubali, 22, of Cambridge; Muhammad Jalloh, 19, of no fixed address, and a 17-year-old boy from Telford, Shropshire, were found guilty of Gomoh’s murder after a trial at the Old Bailey.

The defendants were also convicted of conspiracy to cause grievous bodily harm with intent to another man who got away and has never been identified. Jurors deliberated for almost 15 hours before returning the verdicts of guilty on all counts.

Members of Gomoh’s family attended court in person on Wednesday and gasped as the verdicts were read out.

The trial had heard how Gomoh was picked at random by the Northside Newham gang to send a message to the rival Custom House and Beckton gangs. The defendants armed themselves with knives, changed clothes and got a stolen car with false number plates before setting off to south Newham to find someone to attack.

They first chased an unidentified man who managed to run away. Less than an hour later, Gomoh left his home address in Custom House to go to the supermarket. He was speaking on the phone to his girlfriend, when she heard someone demand “where are you from” and sounds of a struggle.

After inflicting multiple stab wounds, the attackers fled in a stolen car driven by Colubali. A pair of sunglasses left at the murder scene were found to have the 17-year-old defendant’s DNA on them. A second pair of glasses inside the discarded getaway car had Jalloh’s DNA on them.

Police uncovered a series of drawings from Ture’s bedroom showing a number of young men carrying out a stabbing in order to attack one of their rivals. The sketches not only showed the location of Gomoh’s murder in Freemasons Road but the clothes the defendants had worn.

Detectives said the sketches were a “picture-book story of the events leading up to the murder”, and included detail only the killers would have known.

The prosecutor, Oliver Glasgow QC, said: “What had David Gomoh done to deserve such a fate? He had the misfortune to live in an area of London that is frequented by rival gangs.”

He said the murder was not a case of mistaken identity as the attackers did not care whom they killed, only where their victim came from.

The defendants all denied being present at the scene of the killing. Jalloh admitted associating with a gang in his work as a drug dealer. The court heard he had two convictions for carrying knives and that Ture had a conviction for wounding with a knife.

Mark Lucraft QC, recorder of London, thanked jurors for their efforts during the trial, which lasted just over six and half weeks.

A sentencing date was set for 17 September.