Cmdr. Mark Schauer says the tractor was being used to try to pull a stump out of the ground. Investigators say the fire burned a nearby mobile home, where Harrison lived.
Police spokeswoman Julie Garcia says Harrison had been with the Corpus Christi police department since 1990.
Harrison also was an ordained minister and sometimes taught at Cathedral of the Palms Church. Associate pastor Dennis Miller says Harrison was known as "Officer Friendly."