Robert Seman was accused of molesting the girl before killing the family members, according to Mahoning County Sheriff Jerry Greene.
Surveillance footage showed Mr Seman walking with two officers before he jumped off a fourth floor balcony.
He was leaving the courthouse after a status hearing before his trial.
Mr Seman was accused of setting fire to a Youngstown home in March 2015, killing Corinne Gump and her grandparents, Bill and Judy Schmidt.
“We watched the video, and it’s an apparent suicide,” Mr Greene said, noting that Mr Seman was walking without handcuffs on Monday morning with two deputies when he lunged over a balcony and fell to his death inside the building’s rotunda.
“There was no scuffle or anything,” he continued.
Assistant Prosecutor Dawn Cantalamessa said his actions were a sign of guilt.
“I think it’s very telling,” she said.
“He knew the evidence against him. Every witness we talked to in preparation for the case, they didn’t know why he wasn’t pleading guilty or asking for some sort of plea, so this is very telling.”