Police in Philadelphia are working to uncover the motive in a sprawling Monday evening shooting that investigators believe left five people dead and two children injured -- a shooting that ended with the arrest of a suspect authorities say had a bulletproof vest, an AR-15-style rifle and a handgun.

Authorities initially said four people were killed, but early Tuesday announced the discovery of a fifth body in the same Kingsessing neighborhood in southwestern Philadelphia, and that they preliminarily believe that person was killed in the same shooting spree.

The shooting spanned several blocks in the Kingsessing neighborhood, Philadelphia Police Commissioner Danielle M. Outlaw said in a Monday night news conference.

The scenes were littered with dozens of spent shell casings, and several vehicles were damaged by gunfire, she said.

Authorities had yet to name the suspect; Outlaw said she believed he was 40 years old.

When officers arrested the suspect after an on-foot pursuit, he was wearing a bulletproof vest stocked with several ammunition magazines and had the two guns and a scanner, according to the commissioner.

Authorities have "absolutely no idea" why the shooting happened, Outlaw said.

"We're canvasing the area to get as much as we can -- to identify witnesses, to identify where cameras are located, and do everything we can to figure out the 'why' behind this happening," she said.

Investigators have yet to find any connection between the victims and the suspect, Outlaw said.

Of the four slain victims that police initially knew about, three were men ages 20, 22 and 59. The fourth was a male but investigators didn't know his age, Outlaw said.

The fifth slain person -- a man found dead in a Kingsessing neighborhood home early Tuesday, hours after the other victims were found -- was 31, city police Chief Inspector Scott Small said.

Outlaw described the conditions of the injured children -- ages 2 and 13 -- as stable.

A second person also was in custody Monday night -- someone who authorities believe may have returned fire, Outlaw said.

Monday's violence in Philadelphia was among at least six mass shootings in the US in July's first three days, and among at least 341 mass shootings in the nation 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.

"This devastating violence must stop," Philadelphia Mayor Jim Kenney said in a tweet Monday evening.

"My heart is with the loved ones and families of everyone involved, and I send my prayers to the victims," Kenney said.

How the shooting unfolded

At around 8:30 p.m. Monday, police officers were flagged down in the area where the shooting began, and officers discovered multiple gunshot victims, she 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.

Again, as officers were responding to the second shooting scene, more gunshots could be heard on a nearby street, she said.

Eventually, officers found the suspect and pursued him on foot as he continued shooting, Outlaw said. The suspect was finally cornered in an alley and apprehended, she said.

"Thank God our officers were here on scene (and) they responded as quickly as they did," Outlaw said Monday. "I can't even describe the level of bravery and courage that was shown."

Around 12:30 a.m. Tuesday, the body of the slain 31-year-old man was found by his father in the living room of a house in the neighborhood, according to Small, the chief inspector.

The father reported this to an officer who was in the area investigating the spree from hours ealier. Responding medics declared the 31-year-old -- who had been shot several times -- dead shortly after, Small said.

Investigators found seven rifle rounds inside the home, Small said. Because of the home's location and ballistic evidence, investigators believe that person's death is related to the others, Small said.

"We believe now this is the seventh (person shot) and it'll be the fifth person that was shot and killed," Small said.

Authorities will search the home later with a warrant, Small said.

"If this is in fact related to the six individuals that were shot earlier ... we believe we have the shooter" in custody, Small said.