Police continued their investigation at the Carlsbad elementary school Saturday morning, where a man opened fire on the playground Friday.

Friday around lunchtime, the suspect carrying a revolver shot multiple rounds on the playground of Kelly Elementary School. He injured two second grade girls, who were airlifted to Rady Children's Hospital where they were treated for their minor injures.

Detectives recovered several spent bullets and marked the spots of shell casings with pink flags Saturday. They also used metal detectors to find bullets in the grass near the quad and the playground. At least six shell casing have been discovered so far according to police.

"What were trying to confirm is exactly how many rounds he did fire in the course of this crime because that could add up to additional charges," Carlsbad Police Lt. Kelly Cain said. "During the time he's (the shooter) is in the playground, he gets within a few feet of the quad of the school. As he attempts to reload the weapon, the construction workers confront him."

Steven Kane, Mario Contreras and Carlos Partida, of Chula Vista, were on the campus working in a construction area when they heard shots and kids screaming. The three men ran around the building and observed Brendan O'Rourke near the school, firing rounds with a handgun.

As the three approached the shooter, he turned and ran towards his vehicle, which he had parked near the playground, along the east side of the school. As Kane and Contreras chased O'Rourke, he appeared to be attempting to reload his handgun.

Partida got into his own vehicle and drove around the school to the area where the suspect had parked. As Kane and Contreras chased the suspect over the playground fence, Partida struck the suspect with his truck, knocking him to the ground. The three good Samaritans then held O'Rourke down and removed the weapon from him.

Lt. Cain said those men saved several children's lives.

"When you examine the physical evidence you realize how close O'Rourke was to kids. I can't say enough for the three men who took action against an armed-shooting criminal," Lt. Cain said.

Police still do not have a motive for the shooting. They said O'Rourke has not been cooperative with the investigation. The district attorney plans to file charges of six counts of attempted homicide. Detectives said there is no known link between Kelly Elementary school and O'Rourke.

"He never resided in this area and we don't know if he has any family that ever attended this school. For what we can tell this is a random target selection. He accessed the school literally by coming across it and we don't how he picked this school," Lt. Cain said.

Kelly Elementary School will have counselors available on Monday for students and teachers. Carlsbad police plan to have extra officers on campus and around the area during the next few weeks for extra security.

"This is the worst case scenario because it's truly innocent victims. It's the most horrifying kind of crime we see," said Lt. Cain.