By Holly Yan, CNN

(CNN) -- With a photo of a red truck and loose description of the suspect, Texas authorities are begging for the public's help in finding a killer.

For reasons still unknown, the gunman shot 7-year-old Jazmine Barnes in the head as she rode in her mother's car. The devastating circumstances surrounding the young girl's death have united the Houston area in bewilderment and grief.

"It's our belief that it was totally unprovoked, whatever it was, and we're leaving no stone unturned," Harris County Sheriff Ed Gonzalez told reporters. "We're going to leave every motive out there as a possibility."

Jazmine was still in her pajamas Sunday morning when she and her mom, LaPorsha Washington, went on a pre-dawn coffee run to a convenience store. Jazmine's three sisters were in the back seat.

It was about 7 a.m. The family woke up early because Washington's mother was visiting. They were already looking forward to preparing Sunday dinner.

But as the family drove near a Walmart, a man in a red or maroon pickup truck pulled up next to their car and started firing, the sheriff's office said.

The gunfire shattered the mother's driver's side window, Washington told CNN affiliate KTRK. She said the truck's driver then sped up to get in front of the family and continued shooting.

"I didn't provoke him in any kind of way," the mother said.

Gonzalez said the suspect is a white male in his 40s, who had a beard and was driving a red truck.

Washington said she suspects the attack may have been racially motivated. She said she didn't have tinted windows, and the gunman could see "a black mother with four beautiful children, girls, in this car."

The sheriff also said the mother and her four daughters didn't appear to do anything to incite the truck's driver.

"There was nothing to indicate that the family did anything wrong in any way," Gonzalez said. "They were simply just driving along the service road when this happened to them."

The sheriff's department is asking anyone who has surveillance video near Wallisville Road and E. Sam Houston Parkway to check their footage for possible clues.

Even for officers accustomed to horrific crimes, the tragedy of Jazmine's death has made a profound impact on the law enforcement community.

"Help us find this monster before he murders another innocent child," tweeted Joe Gamaldi, president of the Houston Police Officers' Union. "Prayers for Jazmines family, no parent should have to bury a child."

And for Gonzalez, the mission to find Jazmine's killer has gotten personal.

"We're very committed and we're not going to let up. We're extremely upset by this," the sheriff said.

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