Police say that 29-year-old Nathaniel Beller set the fire early Saturday morning.
Three people, including two children, are still in critical condition.
The children's mother, Taniya Johnson, was killed in the blaze.
Investigators think Beller set the fire after an argument, but he hadn't been charged before he died.
There is no word yet on his cause of death.
Two middle-aged and elderly women were also hospitalized in fair-to-serious condition.