Angel Garcia, 33, was pronounced dead at Presence St. Joseph Medical Center in Joliet shortly before 3:30 p.m.
Garcia suffered head trauma and other injuries in the 12:15 p.m. fall, the Department of Corrections said.
Garcia was pulling a 70-year prison sentence for a 2009 Cook County murder. He was scheduled to be paroled in 2079.
Garcia also did prison time for the manufacture or delivery of cannabis, aggravated vehicular hijacking and residential burglary convictions, according to Illinois Department of Corrections records.
Garcia "jumped to his death from the fifth-floor of cell house C," according to a statement from the Department of Corrections said. "The floor is known as '10 Gallery,' as floors are counted by two, in even numbers."
Other sources said the inmate hung himself in the prison. Department of Corrections spokesman Tom Shaer dismissed that claim.
"We vehemently deny that ridiculous claim and I'm sure the numerous witnesses to this incident would deny it as well, as would the seven medical personnel who worked on his head trauma," Shaer said.
Just before jumping to his death, Garcia spent an hour talking to his girlfriend in the visitor’s area, the Department of Corrections said. He reportedly was returning to his cell when he suddenly ran and jumped to his death.
Garcia was not on suicide watch and had not been a mental health patient, according to the Department of Corrections.
"The unfortunate actions and occurrences today did not jeopardize the safety or security of anyone but the decedent," the Department of Corrections said in its statement.