It was unclear Thursday night why the gunman opened fire in the restaurant.
Police are calling that civilian a hero, and crediting him with saving lives.
The man shot the girl and her mother before he was gunned down by another man who witnesses said pulled up in a sport utility vehicle outside the restaurant while patrons and staff took cover.
Two people - "an adult female and a juvenile female" - were shot when the attacker opened fire in the restaurant, and a man broke his leg trying to flee the scene.
The incident comes just weeks after a gunman opened fire at a Waffle House restaurant in Tennessee, killing four people before an un-armed civilian confronted the attacker.
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She said they were going to the restaurant for a birthday dinner for another family member.
"She said they were getting ready to walk in the front door and evidently, the shooter was in the treeline and they were both shot by the front door", recalled Dennis Will.
Two men, 35-year-old Carlos Nazario and 39-year-old Bryan Wittle, had shot and killed the suspect, authorities said.
Shooting suspect Alexander Tilghman walked up to Louie's Grill & Bar, a popular restaurant on the east side of Lake Hefner in Oklahoma City, around 6:30 p.m. local time Thursday and shot multiple times from the entrance. "You are my hero sir". Investigators are working to identify the suspect and his motive. They "have no reason to believe this is a terrorist type of incident", according to police spokesman Capt. "I would think more than likely they would not be filed on", Matthews said.
Matthews says the shooting appeared to be a random act. Mary Fallin, who vetoed a bill that would have eliminated the requirement for people to have completed a firearms training course in order to carry guns in public.