PHILADELPHIA (AP) — A gunman wearing a bulletproof vest opened fire on the streets of Philadelphia on Monday night, killing four people and wounding two boys in the latest outbreak of gun violence in the United States, police said. The victims were apparently random, with no connection immediately known between them and the shooter.

The shootings took place over several city blocks in the southwestern neighborhood of Kingsessing. Responding officers chased the suspect as he continued to fire, and he was arrested in an alley after surrendering, Police Commissioner Danielle Outlaw said at a news conference.

"At this point all we know is that this person decided to leave their home and target individuals,” Outlaw said.

He had a bulletproof vest, an “AR-type rifle,” multiple magazines, a handgun and a police scanner, she said.

Officers were flagged down at about 8:30 p.m., and multiple calls of shots fired came in from Kingsessing. Police found some gunshot victims, and as they were attending to them, they heard more gunfire, Outlaw said.

The suspected shooter was identified as a 40-year-old man. A second person was also taken into custody who may have returned fire at the suspect, but police did not know whether there was a connection between the two people, Outlaw said.

The chief described the scene as covering an area of two by four blocks and said dozens of shell casings were found.

“You can see there are several scenes out here,” Outlaw 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,” Outlaw said.

Three of the dead ranged in age from 20 to 59, while the fourth, who had not yet been identified, was estimated to be between 16 and 21. All were male. The two hospitalized victims are boys, ages 2 and 13. They were in stable condition, Outlaw said.

The shooting occurred a day after gunfire erupted at a holiday weekend block party in Baltimore, about 100 miles (160 kilometers) to the southwest, 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 violence marks the country’s 29th mass killing in 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 killings since 2006, according to the database, in which at least 2,900 people have died and at least 2,000 people have been injured.