Hassan Sentamu, 18, lashed out with a kitchen knife after refusing to hand back belongings of Elianne’s friend, who he had split up with 10 days earlier.
He repeatedly stabbed Elianne outside the Whitgift Centre in Croydon then fled the scene on the morning of last September 27.
Sentamu dumped the blade with Elianne’s DNA on it in nearby Cedar Road and took the number 64 bus towards his home in New Addington, the Old Bailey heard.
Elianne recorded a Snapchat video moments before her death (Image: Met Police) Pc Peter Nolan said he was at the police station when he heard a young woman had been stabbed and a colleague relayed details of the suspect.
Sentamu, then aged 17, was described as wearing a “distinctive” multi-coloured “Trapstar” hooded top and was believed at the time to be the boyfriend or ex-boyfriend of the victim, Pc Nolan said.
Giving evidence on Tuesday, Pc Nolan jurors: “Whilst driving along King Henry’s Drive I saw a 64 bus from Croydon so I decided to stop the bus and see if anyone matched the description on the bus.”
Pc Nolan’s body worn video captured the exchange as he detained Sentamu.
Asked his name, the defendant said he was called “John” before he handed over his Oyster card with his real identity on it.
The defendant asked why, when the officer told him to put his hands behind his back.
The officer then called for “urgent assistance”, informing the defendant: “You are under arrest on suspicion of GBH, stabbing your girlfriend. Where’s the knife? Where’s the knife?”
Pc Nolan told jurors: “All I knew is that a young girl had been stabbed and the officers were doing what they could to keep her alive.
“One of the officers that turned up said to me the female had sadly passed away and it was now murder.”
The officer replied: “I believed what I saw was a smear of blood on his thumb.”
He told jurors he called for evidence bags to put over the handcuffed suspect’s hands before he was put in a van and taken into custody.
Earlier, Pc Alex Smith described the “scene of chaos” when he arrived outside the Whitgift shopping centre on Wellesley Road.
He told jurors: “There were people screaming, shouting, people running away. I knew a serious incident had occurred.”
He grabbed a first aid kit and took over chest compressions on Elianne, who was not breathing.
Pc Ben Carter arrived soon after and used a bandage to apply pressure to Elianne’s neck wound.
He told jurors: “Next to Elianne there was a school bag. Inside was a school planner that had her name on it and that was how we were able to identify her.”
Despite the efforts of members of the public and the officers, Elianne was pronounced dead at the scene at 9.21am.
Jurors watched CCTV footage of the incident in which Elianne was fatally stabbed and images of the defendant running away.
Moments before, Elianne appeared to be caught up in the drama when she recorded a short video clip on Snapchat.
Sentamu, who was studying sports science at Croydon College, has admitted manslaughter but denied Elianne’s murder on the basis of “loss of control” because he has autism.
He denies a charge of having a blade, claiming he had a “lawful reason” for carrying it.
The Old Bailey trial before Mrs Justice Cheema-Grubb continues.
