Philadelphia, Pennsylvania - At least five people were killed and two children were wounded in a mass shooting in Philadelphia Monday night, police said, the latest chapter in the country's gun violence crisis.

Four of those killed were all men aged between 20 and 59, Philadelphia police commissioner Danielle Outlaw told journalists, while the two children wounded were aged two and 13.

A fifth person was found dead by relatives in his home on Tuesday morning and is believed to have died in connection to the shooting.

Police apprehended the suspected shooter – a 40-year-old who was wearing a bulletproof vest and was armed with a rifle and a handgun – near the scene of the shooting in the city's Kingsessing neighborhood, Outlaw said.

"I don't know if he was firing the rifle at the time we were pursuing or the handgun, but yes he was actively shooting a firearm at the time our officers were pursuing," she said.

Earlier local media reports said four people had been hurt in the shooting, but Outlaw put the number of wounded at two, both children.

Local media footage showed police had cordoned off an extensive crime scene, which Outlaw said spanned at least eight city blocks.

"We have absolutely no idea why this happened. Like I said we have this person in custody and we are doing what we can to positively identify this person," she said.

Police said they had also apprehended a second person, who they believe returned fire against the shooter.

"At some point as victims were being shot, we have another person that we believe acquired a gun somehow... and returned fire in the direction of the shooter that we have in custody," said Outlaw.

Over the weekend, two people were killed and 28 wounded in a shooting at a Baltimore block party. Police are still looking for the perpetrator.

Monday's shooting was the 341st that the US has seen this year, according to the Gun Violence Archive, which defines a "mass shooting" as a gun-related incident in which at least four or more people are wounded or killed.