Crossing the playground at lunchtime, 15-year-old Harvey Willgoose had no idea his friend turned enemy Umar Khan had come to school with a hunting knife stuffed beneath his coat.

Nor were there any signs to observers at All Saints High School in Sheffield that a fight was about to break out. Let alone a fatal stabbing.

When police later combed through Khan's phone and devices, however, they found a trail of digital clues that could be followed like a map - as revealed in the first episode of Channel 4's documentary series 'Murder Case: Digital Detectives'.

Khan, also 15, had posed for selfies with weapons, been filmed walking around school with a knife, and demonstrated an increasing obsession with blades through his internet searches.

If identified earlier, these patterns of behaviour could have allowed the school and authorities to prevent a tragedy.

But as it was, when the two boys met in the courtyard on February 3 last year, Khan stabbed Harvey twice with a 13cm serrated blade - once in the chest and once in the abdomen. Harvey was dead within a minute.

Khan was sentenced to life in prison with a minimum term of 15 years after a detailed investigation by South Yorkshire Police.

By piecing together the digital evidence, prosecutors were able to disprove the defence's argument that Khan had momentarily lost control and lashed out - which could have amounted to manslaughter rather than murder.

Detectives identified that Harvey and Khan had over the weekend become embroiled in bitter argument on social media between two friendship groups.

It was sparked by a fight at school the previous week between two pupils - each a friend of one of the boys.

Khan had insisted that Harvey's friend had a knife on him, forcing the entire 1,400-pupil school into lockdown, but police never found one.

Harvey had not been in school that day, and in fact had often stayed at home over the past year out of fears of pupils bringing in knives.

He texted his father two days after the fight, on the Friday: 'Am not going in that school while people have knives.'

The argument intensified and rumbled 'non-stop' over the weekend, with Khan, Harvey and others trading threats of violence over Snapchat, detectives found.

In the early hours of Saturday, Harvey had posted his address onto a group chat and tagged Khan's friend, writing: 'If you've got a problem with [my friends] - any of them - you've got my [address], I'll deal with it, simple.'

Before school on Monday, Harvey had opened up Snapchat again, asking Khan if there was 'beef' [an argument].

He replied: 'nah but if u wan beef we can hav it'. 

Harvey decided to return to school on the Monday - when Khan stabbed him to death.

Another key part of the investigation was interpreting the CCTV footage captured at All Saints High that day.

Dan Dare, an experienced South Yorkshire Police CCTV coordinator who worked on the case, told Channel 4: 'There was a very comprehensive coverage of the school.

'There were about 125 cameras. We collected roughly about 500 hours of footage - so you imagine it takes a lot of time to view. Cameras capture them coming to school. They both arrive late.'

There were four cameras in the courtyard where Harvey was hanging out that lunchtime. One was broken, two were pointing in the wrong direction, but one gave a good view of what came next.

Dare said: 'The challenge was interpreting what you were seeing. Obviously, I expected something had gone off. 

'But when we look at the incident itself, there was nothing pre the courtyard that Umar was about to stab Harvey... The stabbing was very unexpected. And shocking.'

Footage from just after the attack shows Khan inside the dining area looking incredibly agitated, hopping from leg to leg. 

Detectives also found a disturbing cache of pictures on Khan's phone, which prosecutors said showed an 'unhealthy interest in weapons'.

He had taken photos of himself posing with weapons, including a knife and an axe, which he said were to warn others not to mess with him.

Khan had also searched for 'zombie killer knife' and other types of blades online.

On the day of the stabbing, he searched for a prayer that would keep him safe.

Speaking after her son's death, Caroline Willgoose said: 'The house was always really busy, his friends were always there.

'He liked to be at home so our house was always full of kids. Now it's quiet...'

She said Khan had 'ruined his life, and Harvey hasn't got a life anymore,' adding there were now two families 'with an empty bedroom' and there are 'no winners here'. 

Sentencing Khan to life with a minimum term of 15 years, Mrs Justice Ellenbogen told Khan his actions had 'blighted' the lives of students and teachers at the school, adding: 'From the evidence given at trial, it is clear that Harvey was also a popular pupil whom students and teachers held in affection.

'Their lives too have been blighted by your actions, which have affected them deeply, and will continue to do so.'

She continued: 'I'm also sure that you didn't act to any extent in self-defence or fear of violence.'

'Murder Case: Digital Detectives' airs on Channel 4 tonight at 9pm