Alan Cartwright
Alan Cartwright was stabbed while riding a bicycle with his friendsMet Police

Detectives investigating the killing of 15-year-old Alan Cartwright have arrested a third person on suspicion of his murder.

Metropolitan Police said a 21-year-old man has been arrested at an address in Camden, north London, on suspicion of murdering the teenager as he rode with friends through Islington on 27 February.

Two other people, an 18-year-old man and a 17-year-old male, were previously arrested on suspicion of Alan's murder and remain in police custody.

The 15-year-old died after being stabbed in the chest while riding along Caledonian Road. Police released CCTV footage of the "truly shocking" incident in a bid to help find those responsible.

Another two people can be seen on the video pulling Alan's friends off their bikes before the main suspect leaves the scene on the back of one of the stolen bikes.

Following his death, his family appealed for witnesses to come forward with information about the "senseless" attack.

His mother, Michelle Watson, described Alan as a "joker" who would never harm anyone.

She added: "He always made you laugh about anything and everything. He was just the funny one out of his friends. I feel angry, I want somebody found for this. I just feel numb."

Detective Chief Inspector Chris Jones, who is leading the investigation for the homicide and major crime command, said: "Alan, in my mind, was not a gang member and was purely a 15-year-old boy cycling with his friends on a Friday night at 7.30pm."

He added: "It is obvious from the footage that Caledonian Road is busy with cars and pedestrians. We have already spoken to a number of witnesses who have provided helpful information to the investigation; however I would like to re-appeal to anyone who may know something about Alan's murder and the robbery of the bikes."

Police are asking anyone with information to call the incident room on 0208 345 3734 or contact Crimestoppers anonymously on 0800 555 111 or via crimestoppers-uk.org.