William Williams, 59, died in jail on the night of Sept. 14 after jail staff Tased and put him in an isolation cell.
On Thursday, the Snohomish County Medical Examiner ruled the death was homicide.
Williams had been arrested that same night on suspicion of theft for allegedly shoplifting from an Everett gas station.
In September, Aaron Snell with Everett Police said Williams became combative and wouldn't follow jail staff's commands after he was processed into jail, which led to the physical altercation.
"During a normal check, the found the man unresponsive," Snell said after the death. "They started applying CPR to the man, unfortunately the man did pass away."