A police official said the arson was set to cover up the crime scene at Fero's Bar and Grill.
The case's medical examiner did not release the cause of death of any of those killed.
Three men have been arrested for investigation of murder, robbery and murder.
Tips given after a police news conference led to the three suspects. Police believe the three men had been to the bar before.
The bar, in a strip mall south of downtown, drew a mixed crowd of regulars and visitors from nearby hotels, but residents said it was never crowded and some said it was struggling.
The Associated Press contributed to this report.