A police officer has told the Old Bailey how he arrested the killer of 15-year-old Elianne Andam when he saw "a smear of blood on his thumb".
Hassan Sentamu, then 17, stabbed the schoolgirl outside the Whitgift shopping centre in Croydon, south London, in September last year.
She was standing up for a friend who had wanted to get back her teddy bear from him, the jury has been told. Mr Sentamu admits killing Elianne but denies murder on the basis his responsibility was diminished due to his autism.
PC Peter Nolan told the court he had been at the police station when he heard a young woman had been stabbed, and then went out to look for the suspect.