The 911 calls rolled in back to back beginning at 10:12 p.m. Saturday — first reporting a mass gathering of young people outside a Dave & Buster’s, then that someone had been stabbed and then that someone had been shot.

Within minutes, officers pulled into the Ritchie Station Marketplace shopping center in Capitol Heights, Md., and found as many as 100 preteens and teenagers clustered around a scene that had started as a weekend hangout and morphed into a deadly dispute.

One 13-year-old boy had been stabbed and was wounded. A second 13-year-old boy had been shot and killed. And a 12-year-old boy, officials say they later determined, had pulled the trigger.

On Monday, Prince George’s County police announced that the 12-year-old had been arrested in the killing. He is the eighth youth to be charged with murder in Prince George’s County so far in 2021, already surpassing the six juveniles charged with murder all of last year.

Authorities did not release the name of the 12-year-old, who they said was from Washington. The boy appeared before a judge Monday and remains in detention, officials said. In Maryland, children must be 14 or older to automatically be charged as an adult in certain felonies, such as murder.

The shooting victim, 13-year-old King Douglas, died at the scene of his injuries, police said. He was in the eighth grade at Kettering Middle School in Upper Marlboro, Md., the school’s principal said in a letter to parents Monday announcing the boy’s death and offering counseling to students.

Police did not name the 13-year-old who was stabbed. That child’s injuries were not life-threatening, police said; he was treated at a hospital and released.

Maj. Brian Reilly, who oversees homicide investigations for the county police department, said detectives are working to find out what exactly happened Saturday night that led to the stabbing and the shooting.

So far, he said, officers have determined that the shopping center parking lot had become a gathering location for young people in the area during the pandemic. Parents would drop their kids off near the Dave & Buster’s, Reilly said, at least some thinking that the preteens and teens would be hanging out inside the arcade.

Instead, they were clustering in the parking lot, largely unsupervised, Reilly said.

On Saturday night, that lack of supervision led to several “disputes” between the different friend groups gathered there, police said. One group, which included the 12-year-old and the 13-year-old stabbing victim, got into an “argument and physical fight” with a second friend group, to which King belonged.

Reilly said police have “no indication” that King was involved in the stabbing.

At some point, Reilly said, the 12-year-old boy retrieved a handgun from another friend and allegedly fired it. Then the boy returned the gun to the friend, who fled. Police have not been able to recover the gun or locate the friend, Reilly said.

They are also still investigating who stabbed the other 13-year-old.

Reilly said police have no reason to believe that the dispute in the parking lot was gang-related.

“Part of the problem here is that parents dropped off dozens and dozens, if not hundreds, of kids with no supervision,” Reilly added.

Juvenile crime overall has spiked in the year since the pandemic forced schools to go virtual and upended the economic and societal norms that provided structure for children. Across the region, juveniles have been charged with violent crimes, including carjackings, at higher rates than in past years.

“We are aware that we have seen more juvenile crime over the last year during the pandemic,” Reilly said. “And that happened because there might not be as much supervision going on as there once was.”

More juveniles have also been the victims of violent crime, police said.

In a separate case, police announced Monday that they are investigating the slaying of a 15-year-old that occurred earlier this month in Langley Park.

Nelson Ramos, a 10th-grade student at Montgomery Blair High School, was found after officers were called to a wooded area of the Ruatan Northwest Branch Trail at around 3:15 a.m. on April 7.

Officers found the teen off the walking trail, unresponsive and suffering from trauma. Ramos, who lived in Takoma Park, was pronounced dead at the scene.

Police are working to determine a motive in that case.

Magda Jean-Louis and Donna St. George contributed to this report.