Harris, who died Friday, had been in critical condition since the robbery on May 20.
Police saw Harris in flames as she stepped out of the Fina station where she worked.
One officer grabbed a fire extinguisher and began to douse the flames her.
She told the officers she had just been robbed and the robbery had doused her with a liquid and set her on fire.
She gave officers a description of the man.
Officers later arrested Matthew Lee Johnson of Garland after receiving a call of attempted break-ins near the station.
He remains in the Dallas County Jail charged with attempted capital murder in lieu of $1 million bond.
Authorities say they will now pursue a capital murder charge.