As family and friends mourned the death of a 15-year-old boy shot in the head by a police officer in a Dallas suburb over the weekend, the police chief said Monday that new evidence showed the killing did not unfold the way the authorities had originally claimed.

The Police Department in Balch Springs, Tex., said Sunday that the officer, whose name has not been released, fired on a car carrying the teenager, Jordan Edwards, a freshman at Mesquite High School in nearby Mesquite, Tex., because the car was reversing down a street toward the officer in an “aggressive manner.”

But Jonathan Haber, the police chief, told reporters at a news conference on Monday afternoon that video showed the opposite. He said the officer fired when the car was “moving forward as the officers approached,” according to The Associated Press. The Dallas County medical examiner’s report ruled the death a homicide caused by a “rifle wound” to the head.

Lee Merritt, a lawyer for the Edwards family, praised the police chief for his willingness to admit the department’s mistake and called the new account “a big deal.”