Witnesses reported hearing an explosion before just after 7 a.m. in the nine-story, 170-unit Amberson apartment complex in the city's Shadyside section.
The Allegheny County medical examiner has identified the dead man as 60-year-old Mark A. Williams. An autopsy shows he died of external burns and from inhaling superheated air.
Two people from other units in the building had to be treated for smoke inhalation.
County Sheriff William Mullen said Williams was set to receive an eviction notice Thursday. City Fire Chief Darryl Jones confirmed the cause of the blaze and police later in the day confirmed it was caused by Williams.