Police say that the family of four eating lunch inside the restaurant and Schlager briefly exchanged words before he opened fire.
The 29-year-old mother, 33-year-old father and their two sons were all hit by gunfire. The father and his 6-year-old son died from their wounds.
The mother, shot nearly 10 times, remains in critical condition and is undergoing surgery at a local hospital. Her other 5-year-old son also remains in critical condition in the hospital.
"She was able to provide a statement to our detectives," said San Bernardino police Lt. Jarrod Burguan. "We have a lot of facts based on what she was able to tell us."
The identities of the victims, all from San Bernardino, have not yet been released.
After shooting his family, Shclager shot himself in the head. Schlager was still alive when officers arrived. He was taken to Arrowhead Regional Medical Center where he was alive for a short time before succumbing to his injuries.
"When the officers rolled him over, I'm told he had one hand gun that was on the ground right next to him and another handgun that was down around his waist area," Burguan said.
"Both of the male juveniles were transported to the local area hospital trauma center within 16 minutes with critical wounds, gunshot wounds," said Steve Tracey of the San Bernardino Fire Department.
Police say there were several other customers and employees in the business when the shooting occurred. Nobody else was injured. "They were screaming as they were coming out of Del Taco," said Dolored Garcia, who was there at the time. "People here in the parking lot were screaming, 'Gun shots, gun shots.'"
According to authorities, Schlager has a criminal past that extends from 1972 to 1999, including theft, DUI and assault with a deadly weapon. In 2006, someone took out a restraining order against him, but not the victims of today's shooting.
His last-known address was in Lancaster, but he also had an address in San Bernardino.