Texas authorities released a surveillance image of a pickup truck the man suspected of killing a 7-year-old girl was driving the day of the apparent "unprovoked" attack.

Jazmine Barnes, 7, was killed when the unknown suspect pulled up next to her mother's car on Sunday morning and began shooting, Harris County Sheriff Ed Gonzalez said.

Jazmine Barnes, 7, was shot and killed while sitting in a car in Harris County, Texas, on Sunday, Dec. 30, 2018. Authorities released this photo Monday in hopes of finding her killer.(Harris County Sheriff's Office) Jazmine Barnes, 7, was shot and killed while sitting in a car in Harris County, Texas, on Sunday, Dec. 30, 2018. Authorities released this photo Monday in hopes of finding her killer.

Authorities, who are desperate to find Jazmine's killer, have released a photo of the suspect's red pickup truck -- among the 1,300 similar vehicles registered in the area.

"We won't rest until this precious child's killer is brought to justice," Gonzalez tweeted Monday.

Civil rights attorney Lee Merritt and activist-journalist Shaun King have offered a $35,000 reward for the arrest of the gunman.

"The amount started off at $25,000 and we were able to get together $35,000, and we expect if this goes on beyond tonight the number will grow," Merritt told Houston ABC station KTRK. "There are a lot of people who are concerned in our circles. ... This is a tragedy that the community wants to help with in some way. If a reward for the person who shot this poor little girl helps, then we want to offer it."

Authorities in Houston are searching for a red pickup truck caught on surveillance.(Harris County Sheriff's Office) Authorities in Houston are searching for a red pickup truck caught on surveillance.

Jazmine's mother, 30-year-old LaPorsha Washington, was shot and wounded in the incident, Gonzalez said.

Three of Jazmine's sisters were also in Washington's car at the time. Jazmine's younger sister suffered glass-fragment injuries. Her two older sisters weren't hurt, but they were "shaken" and "devastated," authorities said.

Authorities are investigating a shooting in Houston, Texas, that left a girl dead, Dec. 30, 2018. (KTRK) Authorities are investigating a shooting in Houston, Texas, that left a girl dead, Dec. 30, 2018. 
Authorities are investigating a shooting in Houston, Texas, that left a girl dead, Dec. 30, 2018. (KTRK) Authorities are investigating a shooting in Houston, Texas, that left a girl dead, Dec. 30, 2018. 

"I need everyone to pull together to get justice for my child," Washington told KTRK. "She was innocent. She did not deserve this."

The suspect was described as a bearded white man, possibly in his 40s, wearing a red hoodie and driving a four-door, red or maroon pickup truck, officials said.

The sheriff's office urged anyone with information to call 911, tweet at @CrimeStopHOU or call 713-222-TIPS.

ABC News' Enjoli Francis contributed to this report.