GARLAND, Texas (AP) — Three teenagers were killed and a fourth was critically wounded when another teen shot them at a gas station in the Dallas area over the holiday weekend, police said Monday.

The shooting happened Sunday night in the Dallas suburb of Garland and was captured on surveillance video, the Garland Police Department said. Investigators are still trying to determine a motive.

Garland Police Chief Jeff Bryan said a 14-year-old boy who investigators believe is the shooter was arrested Monday afternoon.

In surveillance video police showed during a news conference, a shirtless boy or man could be seen crouching while walking toward the store and carrying a handgun. He swings open the door and starts shooting into the store from the doorway.

Police said he fired more than 20 rounds from a .40-caliber pistol.

Authorities did not name the suspected shooter, who was wearing a baseball hat, blue surgical mask and dark-colored athletic shorts. They said he got into a white Dodge Ram pickup with a scrape along much of the passenger side and someone else drove him away.

Police said they are looking for the person who was driving the pickup.

Bryan said investigators are still trying to determine what led to the shooting.

Two of the victims who died were 16-year-old boys who had arrived at the store together. The third who died was a 14-year-old boy who was there ordering food from the kitchen at the store.

The fourth person who was shot and critically wounded was the cook at the store. He is 15 and was still hospitalized Monday.

Their names have not yet been released.

Bryan said the youth of the victims and the suspected shooter make this difficult for him and his officers to investigate on a personal level.

“I have kids about the same age, and to comprehend the day after Christmas — when most people are winding down through the holidays and getting ready to go back to school — and now we’re trying to support and help families through this,” Bryan said. “It’s difficult to describe in a lot of ways, but we know we have a job to do.”