Detectives are appealing for witnesses after two boys sustained shotgun pellet injuries in Harrow on the afternoon of Sunday, 6 May.
Police were called at around 13:15hrs to High Street, Wealdstone to reports of a person having been shot.
At the scene, a 15-year-old boy [Victim 1] was found to have suffered head injuries, caused by a number of shotgun pellets. He was taken by the London Ambulance Service (LAS) to a central London hospital where his condition has been deemed not life-threatening. He remains in hospital.
Approximately four minutes later, the LAS alerted police to reports of a second injured boy at a different location on the High Street.
A 13-year-old boy [Victim 2, initially believed to have been aged 12 years] - who had been walking along the road with his parents - was found to have a shotgun pellet wound to the head. He was taken to a west London hospital with non-life threatening injuries and has since been discharged from hospital.
The second victim is being treated as an innocent bystander. It is thought that he was struck by shots fired in the shooting minutes earlier.
Officers have also been made aware of a possible third victim who may have been at the scene with the 15-year-old boy. This victim is believed to have sustained an arm injury and left prior to the arrival of the emergency services.
A 39-year-old man was arrested on the evening of 6 May in connection with the incident. He has been released under investigation.
Detectives from the Trident and Area Crime Command are investigating. Enquiries are ongoing.
Anyone with information that may assist police is asked to call Trident officers via 101 quoting reference 3563/6May or tweet @MetCC or contact Crimestoppers anonymously on 0800 555 111.
Detective Chief Superintendent Simon Rose, Harrow Borough Commander said: "It would appear that the first victim was approached by two male suspects, one in possession of a shotgun - and shots were fired injuring him and an entirely innocent member of the public.
"This was a callous, reckless and brazen act, without any thought by those responsible for the fact that there were families with children and people in the high street enjoying their weekend. This was quite simply, appalling.
"As a parent myself, I know hearing an incident like this where a child has been shot whilst they have been with there parents will cause alarm. It is senseless and shows an utter disregard for the safety of others.
"I want to assure the communities of Harrow and the wider public we are doing everything we can to catch the suspects responsible. Violent crime has no place on our streets. Officers from Trident are investigating and we are supporting them locally.
"I would urge anyone who can assist us with the investigation or any witnesses who have not yet come forward to contact police.
"As we have already stated, over the Bank Holiday weekend additional officers are on the streets across the capital, working hard to keep London safe. There are highly visible local patrols, supported by armed response officers, traffic motorcycles, dog units, and air support - alongside plain clothes intelligence-led operations.
"Locally we are flexing our resources to increase the uniform police presence in Harrow, and working with Operation Trident to ensure our proactive effort is tasked in a targeted way. There will be additional patrols in the area of the shooting over the coming days.
"We continue to work with our partners and the local authority to tackle violent crime. Reducing violent crime can only be achieved if we all work together."