The child was found unresponsive Wednesday afternoon in an Austin parking lot when the temperature was 96 degrees, the Austin American-Statesman reported.
Bystanders discovered the baby and tried giving her CPR. She was taken to the hospital where she died about an hour later, Violent Crimes Unit Cmdr. Julie O'Brien said.
Authorities did not say whether those who tried to help had to break into the truck to reach the child, the newspaper reported.
Police, citing a pending investigation, did not disclose how long the baby was left in the car or who left her.
Police will work with the Travis County district attorney's office to determine whether charges will be filed, O'Brien said.
Texas law provides it is a Class C misdemeanor to knowingly or intentionally leave a child unattended in a vehicle for more than five minutes. If the child dies, the crime could be raised to a felony of child endangerment or criminally negligent homicide.