Schlager's stepdaughter, Roxanne Trujillo, 29, and her 5-year-old son were both severely injured. Her condition has been upgraded to stable while the boy remains in critical condition but is expected to survive.
The motive for the shooting is still unclear.
Friends say there were growing problems between the suspect and his stepdaughter.
"The husband told her to tell her stepdad to stay away for a little while, "said Kelly Walker, a neighbor at the family's San Bernardino apartment. "Let him cool down, things could simmer off and maybe he could start seeing the kids again. I think it was just a jealousy thing. "
At Schlager's home in Lancaster, neighbors say he kept to himself, but they would often see the stepdaughter and the two boys at the home.
On Sunday, a small shrine of candles, flowers and teddy bears sat in front of a shattered window at the restaurant.
The shooting happened around 1 p.m. Saturday at a Del Taco on Sterling and Highland avenues. From all accounts, it was a terrifying experience for everyone at the restaurant.
Police say that Schlager walked into the restaurant and went straight to the table where the family was sitting. Police say that he said a few words to them and then shot them multiple times almost at point blank range.
There were several customers and employees inside the fast-food restaurant when the gunfire started.
"They were screaming as they were coming out of Del Taco. People here in the parking lot were screaming, 'Gunshots, gunshots,'" said witness Dolores Garcia.
"The mother who was wounded in this was able to provide a statement to our detectives," said San Bernardino police Lt. Jarrod Burguan. "We have a lot of facts that came out based on what she was able to tell us."
Authorities say that Schlager was armed with two semi-automatic handguns and he had an extensive criminal history.
His wife, the mother of the woman he shot, died several years ago.