A third boy has been arrested in connection with a double shooting in Neasden which left two teenagers seriously injured.
The 15-year-old was arrested at his home this morning and is being held in a north London police station.
Two other youngsters, aged 14 and 15, were arrested at their Brent homes on Monday and are being held in police custody.
Officers were called at 10.49pm on Saturday to reports of shots being fired in Blackbird Hill, Neasden.
Officers and London Ambulance attended and found two men suffering gunshot wounds to the torso.
The victims, who are white and aged 18 and 19, were taken to separate London hospitals where they are in a serious but stable condition.
It is believed the victims were targeted by a group of teenage boys who shot at them and tried to rob them.
Detective Inspector Mick Millar, said: "The two victims were out socialising with three friends in Blackbird Hill, close to the McDonald's restaurant at around 22:30 when the incident took place.
"We understand that the victims were approached by possibly five black youths believed to be in their teens and that a verbal confrontation took place between them. The suspects were aggressive towards the victims and attempted to rob them.
"One suspect produced a handgun and fired several shots at the victims from a close proximity."
The five suspects fled the scene on bicycles, one of which is described as a red 'BMX' type bike.
Mr Millar added: "This was a completely unprovoked attack on two individuals who were out enjoying their Saturday evening with their friends. I believe that there would have been a number of people out on the night, and I urge the to come forward and speak to us if they have any information."
Anyone with any information is urged to call police on 0208 733 4774 or Crimestoppers anonymously on 0800 555 111.