He was discovered in his cell by corrections officers at the Souza-Baranowski Correctional Center in Shirley, Mass., around 3 a.m., the Massachusetts Department of Correction said in a statement.
Lifesaving techniques were attempted, and he was transported to a hospital, where he was pronounced dead at 4:07 a.m., the department said.
“Mr. Hernandez hanged himself using a bedsheet that he attached to his cell window,” the statement said. “Mr. Hernandez also attempted to block his door from the inside by jamming the door with various items.”
Mr. Hernandez was housed in a single cell in the general prison population, the statement said. The assistant deputy commissioner of communications, Christopher Fallon, told The Associated Press that officials had not been concerned that Mr. Hernandez was a suicide risk. He also said no suicide note had been found. The case will be investigated by the state police.
Mr. Hernandez’s suicide came on the day some of his former teammates would be visiting the White House to celebrate their recent Super Bowl victory.
Mr. Hernandez, 27, was sentenced to life in prison without the possibility of parole for the murder of Odin L. Lloyd, who was dating the sister of Mr. Hernandez’s fiancée, Shayanna Jenkins. He had appealed the verdict.
The body of Mr. Lloyd, a 27-year-old semiprofessional football player, was found, shot six times, in June 2013 in a pit at an industrial park near Mr. Hernandez’s home in North Attleboro, Mass. Mr. Hernandez’s motive in that case, prosecutors said, was that Mr. Lloyd spoke with people Mr. Hernandez did not like at a bar in Boston.
Mr. Hernandez was found not guilty just last Friday in a second murder case, a drive-by shooting of two people in Boston in 2012.
Mr. Hernandez was born on Nov. 6, 1989, and grew up in a tough neighborhood in Bristol, Conn., and played at the University of Florida, where he was chosen the country’s best tight end. He also ran into trouble there, getting into a bar fight and testing positive for marijuana.
He was a fourth-round N.F.L. draft pick by the Patriots, after many teams passed on him, leery of his reputation.
Teamed with another star, Rob Gronkowski, Mr. Hernandez gave the Patriots a tight end combo that was second to none. He played three years in the N.F.L., catching 175 passes for 18 touchdowns. He played in one Super Bowl, in 2012, catching a team-high eight passes in a 21-17 loss to the Giants. He signed a $40 million contract extension in 2012 to continue playing for the Patriots.
In 2013, Mr. Hernandez and Mr. Lloyd began dating sisters, and the two men socialized together. In June 2013, at a Boston nightclub, Mr. Hernandez became angry with people Mr. Lloyd was talking with. Investigators said that Mr. Hernandez might have suspected that Mr. Lloyd was talking with them about the double murder case that Mr. Hernandez was eventually acquitted of.
Though no murder weapon was found, and no witness to the shooting of Mr. Lloyd came forward, prosecutors built a circumstantial case tying Mr. Hernandez to the killing.
Mr. Hernandez’s fiancée, Ms. Jenkins, spoke of a box that she said Mr. Hernandez instructed her to remove from their house and discard the day after Mr. Lloyd’s body was found. She also said Mr. Hernandez called her from the police station and asked her to give some money to Ernest Wallace, a friend who would eventually also be charged in the killing.
It was enough for the jury, which deliberated for six days before finding Mr. Hernandez guilty.
Mr. Hernandez’s death means his murder conviction is expected to be vacated under a centuries-old legal doctrine enshrined in Massachusetts’ criminal case law.
“Aaron Hernandez goes to his death an innocent man under the eyes of the law,” said Martin W. Healy, chief legal counsel for the Massachusetts Bar Association, adding, “It’s as if the case never existed.”
Under the doctrine, known as “abatement ab initio,” criminal convictions are essentially nullified if a person dies before they have a chance to complete the process of appealing it, Mr. Healy said. ”The principal behind this legal doctrine is, the individual has not had the ability to clear their name,” he said.
The little-known doctrine has erased several well-known criminal convictions over the years. When Kenneth L. Lay, the chief executive of Enron, died of a heart attack in 2006 before his sentencing for fraud, the doctrine erased his criminal record and meant the government lost access to $44 million he was supposed to forfeit for restitution.
Mr. Hernandez had a daughter with Ms. Jenkins, Avielle, now 4. He has an older brother, D.J.
The Souza-Baranowski Correctional Center, where Mr. Hernandez was an inmate, is a maximum-security prison that has reported a number of problems since it opened in 1998.
It is the prison where John J. Geoghan, a Catholic priest accused of molesting scores of children, was strangled by another inmate in 2003. In January, nearly 50 prisoners were involved in a violent disturbance, damaging sprinklers and computer systems, before prison officials regained control.
“It is a troubled prison we’re very concerned about,” said Leslie Walker, the executive director of Prisoners’ Legal Services of Massachusetts.
Ms. Walker said it was “hard to tell” whether Mr. Hernandez should have been on a suicide watch. “Five days after a series of acquittals, I don’t think many people would be as concerned about his mental health as they might have been, say, during the trial,” Ms. Walker said.
A member of the Patriots’ public relations department, who declined to give her name, said most of the staff was out of the office for the team’s visit to the White House. “We are aware of the reports,” she said, “but I don’t anticipate that there will be a comment today.”
Jose Baez, who defended Mr. Hernandez in the double murder case, told Reuters: “There were no conversations or correspondence from Aaron to his family or legal team that would have indicated anything like this was possible. Aaron was looking forward to an opportunity for a second chance to prove his innocence.”
Mr. Hernandez’s former agent, Brian Murphy, cast doubt on the manner of death. “Absolutely no chance he took his own life,” Mr. Murphy posted on Twitter. “Chico was not a saint, but my family and I loved him and he would never take his own life.”
Mike Pouncey, a Florida teammate now with the Miami Dolphins, wrote on Instagram: “To my friend my brother! Through thick and thin right or wrong we never left each other’s side. Today my heart hurts as I got the worse news I could have imagined.”