Jazmine was with her mother, LaPorsha Washington, and her sisters, including a 6-year-old and two teenagers, the authorities said. Ms. Washington, who was injured in the shooting, told CNN she did not see the gunman but her teenage daughter described him as a white man with blue eyes.

“This just went down very quickly,” Sheriff Gonzalez said. “When the gunfire erupted, we are talking about small children, they witnessed something very traumatic. And it’s very likely the last thing they did see was that red truck and the driver in that truck.”

Research has shown that stress levels and conditions at the time of a crime can undercut the accuracy of eyewitness identification. The sheriff and the family said the sun had not yet risen when the shooting happened.

Image Eric Black Jr., 20, has confessed to taking part in Jazmine’s killing, the authorities said. Credit... Harris County Sheriff's Office

“Eyewitness testimony is the least reliable evidence you can have,” said Lori Brown, a criminologist at Meredith College in North Carolina, who said that people generally try to understand how a traumatic event could have happened by using what they know about the world. “Unfortunately,” she said, “we fill in the gaps.”

After Jazmine’s killing, the public mobilized to help the family. On Saturday, the day before the arrest was announced, nearly 1,000 people gathered at a rally in Houston, clutching banners and shouting for justice for Jazmine, who was in second grade at a Houston-area school.

DeAndre Hopkins, a wide receiver for the Houston Texans, had pledged to donate his paycheck from this weekend’s playoff game, which amounts to $29,000, to help pay for Jazmine’s funeral. And Mr. King, a prominent racial justice activist and a columnist at The Intercept, had raised a $100,000 reward for information leading to the gunman’s arrest.