Correction officers discovered Hernandez in his cell at the Souza Baranowski Correctional Center in Shirley, Massachusetts. He was in a "single cell in a general population housing," the department said in a statement.
"Mr. Hernandez hanged himself utilizing a bedsheet that he attached to his cell window," according to the statement. "Mr. Hernandez also attempted to block his door from the inside by jamming the door with various items."
Correction officers attempted lifesaving techniques on Hernandez, the department said in the statement. He was subsequently transported to the UMass Memorial–HealthAlliance Hospital in Leominster, where he was pronounced dead at 4:07 a.m. ET by a physician.
Hernandez's next of kin have been notified.
Law enforcement sources told ABC News that Hernandez was found with the bible verse John 3:16 written across his forehead. That verse reads, "For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life."
The Massachusetts State Police and the Worcester County District Attorney's Office are investigating his death. An autopsy is being performed in Boston.
From some who knew Hernandez, there was skepticism about the circumstances of his death and whether he killed himself.
Hernandez's former agent, Brian Murphy, said there was "absolutely no chance he took his own life."
"Chico was not a saint, but my family and I loved him, and he would never take his own life," Murphy said.
Hernandez's lead attorney, Jose Baez, said "there were no conversations or correspondence from Aaron to his family or legal team that would have indicated anything like this was possible."
"The family and legal team is shocked and surprised at the news of Aaron's death," Baez said in a statement. "Aaron was looking forward to an opportunity for a second chance to prove his innocence. Those who love and care about him are heartbroken and determined to find the truth surrounding his untimely death."
Baez said that his law firm is requesting that authorities conduct a "transparent and thorough investigation" and that it will participate in its own examination.
The Massachusetts Department of Correction did not immediately react to these statements other than to reiterate that the investigation is ongoing.
The state medical examiner's office has taken custody of Hernandez’s body.
Police and prosecutors who are now investigating the circumstances of the death are looking at Hernandez's recent time in prison, including prior fights and any gang-related disputes he may have had.
Hernandez and Jenkins-Hernandez have a 4-year-old daughter.
Thomas Quinn — the district attorney of Bristol County, where Hernandez was convicted in 2015 of killing Lloyd — issued a brief statement, saying, "This is a shocking and sad end to a very tragic series of events that has negatively impacted a number of families."
An Ohio State University spokesman said it does not anticipate a statement at this time from its head football coach, Urban Meyer, who coached Hernandez at the University of Florida.
ABC News' Dee Carden, Matt Foster and Amanda Maile contributed to this report.
This is a developing story. Please check back for updates.