Sheriff Ed Gonzalez of Harris County said investigators were continuing to look into the fatal shooting of 7-year-old Jazmine Barnes and that they will “continue to do every effort possible” to bring closure to her family.

In the days after Jazmine Barnes, 7, was fatally shot inside a moving car near a Walmart outside Houston on Sunday morning, the police have released just one concrete clue about her killer: an image of a red pickup truck recorded by a surveillance camera near the scene of the shooting.

The authorities believe that the gunman, described as a man in his 40s with a beard and wearing a hooded sweatshirt, was driving the truck. The Harris County Sheriff’s Office believes the gunman fired at random into the car in which Jazmine was riding with her mother and three sisters, a 6-year-old and two teenagers. A bullet struck Jazmine in the head, her mother said.

The police said Wednesday that they still had not identified the gunman. But the case has drawn the attention of national civil rights activists and fueled speculation that the shooting was racially motivated. The gunman is white, the police said, and Jazmine was black.

At a news conference on Wednesday afternoon, Harris County Sheriff Ed Gonzalez said officers were pursuing a number of tips they had received, and taking a fresh look at a similar — and still unsolved — shooting that took place nearby in 2017. The department was also increasing patrols amid concerns that the gunman could strike again.

“We’re not going to stop until we solve this case,” Sheriff Gonzalez said.

Jazmine’s mother, LaPorsha Washington, and her sisters met with Harris County investigators on Wednesday morning to help them create a composite sketch of the gunman, said Lee Merritt, a lawyer for the family.

In another effort to garner the public’s help, Mr. Merritt and Shaun King, a prominent racial justice activist and a…