Michel Veillette, 34, had pleaded not guilty to 13 counts of aggravated murder and two counts of aggravated arson and was awaiting trial.
"Obviously, he was facing the death penalty, multiple counts of murder for the deaths of his wife and children, so it's hard to tell what was going through his mind, and I wouldn't want to speculate," said Warren County Prosecutor Rachel Hutzel.
A corrections officer found Veillette hanging by a sheet, Sheriff Tom Ariss said.
Prosecutors alleged that Veillette stabbed his wife, Nadya Ferrari-Veillette, 33, after they had a fight on Jan. 11. He then set fire to the couple's home in Mason, about 20 miles northeast of Cincinnati, prosecutors said. The children - ages 8, 4 and 2-year-old twins - died of smoke inhalation.
Veillette told The Cincinnati Enquirer that he killed his wife after she attacked him with a knife and frying pan. He claimed that she set the fire.
His lawyer, Tim McKenna, said he last visited his client on March 26. "He had been progressively more upbeat and he was very interested in defending himself," McKenna said.