San Bernardino County Sheriff Shannon Dicus said at an afternoon news conference that Savannah Graziano was wearing tactical gear as she ran toward deputies while the shootout was unfolding.
The teenager was taken to a hospital, where she was pronounced dead shortly before noon.
Authorities did not specify whether the teen was struck by bullets fired by her father or responding deputies.
Her father, 45-year-old Anthony John Graziano, was pronounced dead at the scene. He allegedly killed his estranged wife in a domestic violence incident on Monday in the city of Fontana.
WATCH: San Bernardino County sheriff provides update after murder suspect killed in shootout on 15 Fwy
San Bernardino County sheriff provides update after murder suspect killed in shootout on 15 Fwy
Investigators had issued an Amber Alert after Graziano fled with his daughter, Savannah, in a white 2017 Nissan Frontier with California plates. Graziano was described as armed and dangerous.
The sheriff's department said it received a call Tuesday morning about a vehicle matching the description of the Amber Alert that was triggered Monday for the teen. The call stated the vehicle was traveling southbound on the freeway in Barstow.
Authorities caught up to the vehicle at the Main Street exit in Hesperia and there was an exchange of gunfire between a suspect and deputies. Witnesses of the shootout reported hearing a massive exchange of gunfire.
Deputies had been in pursuit of the father's pickup truck, and multiple shots were fired out of the rear window during the chase.
The vehicle became disabled around Hesperia, and the firefight ensued. Dicus said the girl was wearing tactical gear as she exited a truck's passenger side and ran toward the sheriff's deputies. The deputies did not initially realize it was the girl who was running toward them, Dicus said, because she was wearing a helmet and a military-style vest that can hold armored plates.
Even though the only weapon found was in the truck with Graziano, detectives say it's possible his daughter was also firing.
"There may be some information that the passenger was involved in firing back at the deputies and we're still trying to confirm that at this point," Dicus said.
Tuesday's crime scene caused major backups along Interstate 15. The southbound 15 was closed at Main Street.
The search for Graziano began Monday after his wife was shot and killed near a Fontana elementary school, which triggered panic among parents and schoolchildren nearby. At the time, he was believed to be with his daughter.
"The man just came down the street shooting. He started shooting in that direction," neighborhood resident Andy Davis told Eyewitness News Monday. "Hit those trees, houses across the street. They say the bullets were skipping off the street, and the woman was trying to run for her life, and unfortunately she was hit. Once she was hit I guess he shot at her a few more times."
An Amber Alert was issued for the teen shortly after the killing, but according to the California Highway Patrol, the alert has since been deactivated.
Murder suspect killed in shootout with deputies on 15 Fwy in Hesperia; Amber Alert deactivated
Romero said Graziano is a Fontana resident but investigators learned he previously lived in Arizona.
The Associated Press contributed to this report.