Eric Larson died from blunt force head trauma, and his death was ruled a homicide, the medical examiner said.
Prosecutors said the 25-year-old man accused in the assault, Gabriel Vargas-Garcia, had been released from jail 32 hours earlier after completing a 15-month sentence.
Vargas-Garcia, who has been diagnosed with schizophrenia, was charged July 13 with attempted first-degree murder and first-degree assault, but the case is now being investigated as a homicide, according to the Bellevue Police Department.
Vargas-Garcia is accused of beating Larson, an amputee who used a wheelchair, for 90 seconds before boarding a bus, according to charging documents.
Police arrested Vargas-Garcia two hours later after receiving a report from a woman who spotted him based on a police description of the assault suspect.