The police report and the image it conjured quickly went viral on social media: A 12-year-old Bridesburg boy wielding a semiautomatic assault-style rifle during a neighborhood beef on the streets of Northeast Philadelphia.

Police on Tuesday said that weapon, and a second retrieved from the boy's 19-year-old brother on Saturday, appeared to have been purchased legally.

Still, both brothers were in custody, facing charges in a dispute that ended without violence but raised concerns about how a preteen ended up armed with an AR-15-style rifle — a weapon that has stoked the national debate about kids, guns, and violence.

"We've had 12-year-olds arrested with guns" before, said Lt. Dennis Rosenbaum of Northeast Detectives. But this case, he said, is "just an odd story all around."

According to Rosenbaum, the incident unfolded like this:

About 6:30 p.m. Saturday, 19-year-old Michael Mitchell took his mother's handgun, went across the 2200 block of Kennedy Street with his younger brother,  and knocked on a neighbor's door after some kind of dispute with a teen who lived there.

Michael Mitchell
Philadelphia Police
Michael Mitchell

The teenager's father opened the door, saw Mitchell with a handgun, and asked what he was doing. Mitchell then yelled to his younger brother: "Go get my gun!"

The 12-year-old, whom police aren't identifying by name, grabbed his brother's Diamondback 15 rifle from their home, came back outside, and pointed it at his neighbors, Rosenbaum said. The gun was loaded with about 20 rounds of ammunition.

Neighbors called 911.

When 15th District Officer Krzysztof Wrzesinski, who was on patrol nearby, drove up, the two brothers put their guns in a car on the block and complied with the officer.

"He took them into custody, got the guns," Rosenbaum said.

Police are investigating Mitchell's claim that he legally owned the semiautomatic rifle, which police say retails for about $800 or $900. Mitchell wouldn't answer officers' questions about why he had the gun, but he has no criminal history that would have barred him from buying it, Rosenbaum said. The Bureau of Alcohol, Tobacco, Firearms and Explosives is tracing the weapon's origin, he said.

Under state law, anyone who's at least 18 can buy a semiautomatic rifle if he or she passes a criminal background check, has no protection-from-abuse orders, and was not involuntarily committed for mental-health reasons, Rosenbaum said.

Police on Monday tweeted a photo of Wrzesinski with the seized weapons. (Police initially said the older brother was 18.)

The mother of Mitchell and the boy is cooperating with police, Rosenbaum said. She said that she had her handgun locked in a safe and that her 19-year-old son took it without permission, he said. "She's furious" about what happened, he said.

The ATF will also be conducting a trace report on her handgun, which had four rounds in it, Rosenbaum said. Such reviews could take weeks or months, he said.

Mitchell was charged with gun offenses, terroristic threats, corruption of a minor, and related offenses. Bail was set at $55,000 and he remained in custody Tuesday at the Curran-Fromhold Correctional Facility. Mitchell faces a hearing next Wednesday on the charges; his court-appointed attorney, Dennis Turner, declined to comment Tuesday.

His brother was charged as a juvenile with similar offenses, Rosenbaum said. He has a hearing scheduled for April 19. It was not known Tuesday if he was still in custody or released. It also was not known if the younger brother has an attorney.