A gunman wearing a bulletproof vest opened fire in Philadelphia on Monday night, killing four people and wounding two others, police said.

Police Commissioner Danielle Outlaw said at a news conference that all the victims were male.

The suspect was arrested in an alley without incident, Outlaw said. He had 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,” Outlaw said.

The shooting occurred a day after gunfire erupted at a holiday weekend block party in Baltimore, about 100 miles 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 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.