Four people have been killed in a shooting in Philadelphia, police have confirmed.

Police commissioner Danielle Outlaw said two boys, aged two and 13, were in a stable condition after being injured in the shooting on Monday and a suspect was in custody.

She said all of the victims were male – three aged from 20 to 59 with the fourth believed to be between 16 and 21 – and the suspect was arrested in an alley without incident with a bulletproof vest, multiple magazines, an AR-type rifle, a handgun and a police scanner.

She said: “At this point all we know is that this person decided to leave their home and target individuals.”

The suspected gunman was identified as a 40-year-old man with one other person in custody.

The commissioner, who said dozens of shell casings were found, said police were attending to victims when they heard more gunfire.

“There are several scenes out here,” she said. “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.”

The shooting occurred a day after gunfire erupted at a holiday weekend block party in Baltimore, about 100 miles to the south west, killing two people and wounding 28 others.

The wounded in that shooting ranged in age from 13 to 32, with more than half of them minors, according to officials.

The Philadelphia incident is the 29th mass killing in the US during 2023, according to a database maintained by The Associated Press and USA Today in partnership with Northeastern University.

So far this year, the nation has witnessed 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, according to the database, in which at least 2,900 people have died and at least 2,000 people have been injured.