A heavily armed gunman wearing a bulletproof vest opened fire in southwest Philadelphia on Monday evening, killing four people and injuring two others, the police told local media.

A male suspect was taken into custody by the police just before 8:40 p.m., the authorities said, adding that they had recovered a rifle, a handgun and another gun in the alleyway behind the 1600 block of South Frazier Street.

At least two of the people shot were juveniles,who were stable, the police said, adding that all of the victims were taken to the Penn Presbyterian Medical Center, and that the two children were expected to be transported to Children’s Hospital of Philadelphia.

Earlier reports suggested that four people had been shot and injured.

A second person who police believed had “picked up the gun and returned fire in the direction of the shooter,” was also in custody, Danielle M. Outlaw, the police commissioner, said at a news conference late Monday night.

According to the police, an officer heard gunfire in the Kingsessing neighborhood just before 8:30 p.m. and found at least one victim near South 56th Street and Chester Avenue.

The police also received “multiple reports of a male armed with a rifle” who was shooting in that area, according to The Inquirer. No officers discharged their firearms during the arrest, the police said.

Katherine Gilmore Richardson, a member of the Philadelphia City Council, said on Twitter that she was “deeply saddened to learn of the mass shooting that took place this evening.”

In a statement released on Monday evening, the Philadelphia Federation of Teachers called on elected officials to act on gun reform.

”Again and again, the devastation wrought by the scourge of gun violence continues to rip families apart and leave communities shattered,” Jerry Jordan, the president of the federation said in a statement.