By Jason Hanna, Michelle Watson and Jennifer Feldman, CNN
(CNN) — A gunman wearing a bulletproof vest opened fire along several blocks in southwestern Philadelphia before police apprehended him on Monday evening – a shooting that’s left four people dead and two children injured, authorities said.
A second person who investigators believe picked up a gun and returned fire also was in police custody following the violence in Philadelphia’s Kingsessing neighborhood, Philadelphia Police Commissioner Danielle M. Outlaw said.
Investigators don’t immediately know what led to the shooting, in which several vehicles were damaged and officers were running after the first suspect even as he was firing, Outlaw said.
“We have absolutely no idea why this happened. … But thank God our officers were here on scene and responded as quickly as they did,” Outlaw said.
The names of neither the detainees nor those shot were immediately released. Three of the four killed ranged in age from 20 to 59, police said. The age of the fourth wasn’t immediately available.
Outlaw described the conditions of the two children who were shot – ages 2 and 13 – as stable.
The first suspect is a 40-year-old man who had an AR-style rifle and a handgun, said Outlaw, who did not have an age for the second detainee.
Earlier, police said eight people had been injured.
At least 340 mass shootings have happened in the US so far this year, according to the Gun Violence Archive. The archive, like CNN, defines a mass shooting as one in which four or more people are shot, not including the shooter.
Officers became aware of Monday’s shooting around 8:30 p.m. when they were flagged down at an intersection and found some gunshot victims, Outlaw said.
“As they were scooping up victims and preparing them for transport to the hospital, they also heard multiple gunshots up the street,” Outlaw said.
“At some point they locate the suspect, and they were able to pursue this male who was on foot. The officers were on foot as well, and they followed or pursued this male,” she said.
Officers eventually apprehended that suspect in an alley.
“When they did, this male was wearing a bulletproof vest with multiple magazines in the vest. He also had a scanner and an AR-style rifle and a handgun underneath his body,” Outlaw said.
About 50 spent shell casings littered the shooting scenes, she said.
“At this point, we don’t see a connection to any of the victims and the shooter,” Outlaw said.
“We’re canvassing the area to get as much as we can to identify witnesses, to identify where cameras are located, and do everything that we can to figure out the ‘why’ behind this happening,” Outlaw said.
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CNN’s Danny Freeman and Sara Smart contributed to this report.