It's unclear how a Denver police officer involved in the deadly shooting of a 17-year-old girl was hurt, but an academic expert on police use of force says the threat perceived by the officer is critical in determining whether the shooting was justified.

Police Chief Robert White said Thursday the officer might have been injured trying to get out of the way of the stolen vehicle. The possibility raised during a preliminary investigation of Monday's shooting clouded White's initial statement that two of his officers opened fire after one was struck by the car.

The shooting occurred after the officers found Jessica Hernandez and four other teenagers inside the car in an alley. White said the officers told the teens several times to get out of the vehicle.

Geoffrey Alpert, a University of South Carolina expert on police use of force, said the threat perceived by an officer when he fires is more important than the way he is hurt.

"It's a very quickly moving situation, it's very fluid, and you have to look at it from the eye of the officer as things are developing," Alpert said. "Then you can evaluate the righteousness of the use of force based on those facts."

White would not comment further on Thursday about the sequence of events or what prompted the officers to fire, stressing that the investigation is in its early stages.

A passenger in the car, who spoke to The Associated Press on condition of anonymity because of safety concerns, has disputed the official account, saying officers came up on the car from behind and fired four times into the driver's side window.

The passenger also said the officers did not yell any commands before they fired, and that the car struck the officer after Hernandez was shot and lost control of the vehicle.

Department policy encourages officers to move out of the way of a moving car rather than use their firearm. But it also allows them to shoot if they have no other reasonable way to prevent death or serious injury.

White said he cannot judge whether Officers Daniel Greene and Gabriel Jordan acted appropriately until criminal and internal investigations are completed. Jordan suffered a fractured leg during the incident.

The shooting was the fourth time in seven months that a Denver officer fired at a moving vehicle after perceiving it as a threat. The incidents have prompted the department and an independent monitor for the city to review policies and training related to such shootings. White said his review will study cases over the past two years.

The U.S. Supreme Court has said officers may not use deadly force to stop a fleeing suspect unless the person is believed to pose significant physical harm. Still, policies vary among agencies, and some departments have banned or discouraged the practice.

Experts say shooting and disabling a driver can send a car out of control.

"If you were to shoot at the driver, you would have an unguided missile, basically," said Chuck Wexler, executive director of the Police Executive Research Forum.