A gunman wearing a bulletproof vest opened fire on the streets of Philadelphia on Monday night, killing four men 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 the 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. One other person was in custody, but no other details were immediately available.

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

“At this point 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.

The dead men ranged in age from 20 to 59. The two hospitalized victims are boys, ages 2 and 13. They are in stable condition, Outlaw said. There didn't appear to be a connection between the shooter and the victims.

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.