A suspect is in custody after four people were killed in a mass shooting in Pennsylvania.

The suspect opened fire in the Kingsessing area of Philadelphia on Monday night in what’s been identified as the 29th mass killing in the US of this year.

The four victims have not yet been publicly identified but police commissioner Danielle Outlaw said they were all male, three aged 20-59 with one aged between 16 and 21.

Two boys, aged two and 13, were injured during the shooting and are currently in a stable condition in hospital.

Dozens of shell casings were found, and officers heard more gunfire as they attended to victims.

The suspect was arrested in an alleyway at the scene. He had a bulletproof vest, multiple magazines, an AR-type rifle, a handgun, and a police scanner.

The suspected gunman has been identified as a 40-year-old man, and there is another person in custody in connection with the attack.

Commissioner Outlaw said: ‘At this point all we know is that this person decided to leave their home and target individuals.

There are two people in custody (Picture: Drew Hallowell/Getty Images)

Two children were injured in the attack (Picture: EPA)

The suspected gunman was arrested at the scene (Picture: EPA)

‘There are several scenes out here. We’re canvassing the area to get as much as we can, to identify witnesses, to identify where cameras are located and to do everything to figure out the why.’

So far in 2023, the US has seen the highest number on record of mass killings and deaths to this point in a single year.

There have been more than 550 mass killing incidents since 2006, in which at least 2,900 people have died and at least 2,000 people have been injured.

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‘We’re canvassing the area to get as much as we can, to identify witnesses, to identify where cameras are located and to do everything to figure out the why.’