The suspect is still on the run in a red pickup truck that fled the scene.
Harris County Sheriff Ed Gonzalez said his detectives are working around the clock to find the person who killed the Sheldon Independent School District second-grader.
Police said that some kind of altercation in the parking lot may have led to the shooting, and the gunman fired into the vehicle holding the five females, killing the girl and injuring the woman in her arm. Jazmine's 6-year-old sister was also treated for injuries she suffered from shattered glass.
LaPorsha Washington, 30, was leaving a Walmart parking lot in Houston at around 7 a.m. with four children in her vehicle, according to Harris County's Sheriff's Office.
"Please keep this family in your prayers", Gonzalez tweeted Sunday night.
Sheriff's Maj. Jesse Razo said the other girls who were inside the auto when the shooting happened were "shaken" and "devastated". "A total of 5 occupants in the vehicle witnessed this innocent child, their loved one, shot and killed before their eyes".
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"People who usually commit this type of crime talk about it", Sheriff's Maj.
The suspect, who is still at large, "is considered armed and risky", Razo told reporters.
The motive for the shooting was not immediately clear.
'We're going to use all resources available to bring this killer to justice, and I urge you, whoever did this, you know who are, please turn yourself in now, ' Razo said.
The sheriff's office is asking locals in the area around Wallisville Road near the E. Sam Houston Parkway, near the Walmart, to check surveillance video to try an identify the suspect. He was described as being in his 40s and having a beard, Razo said.