He was rushed to the hospital at 3:05 a.m., but was pronounced dead at 4:07 a.m.
The former New England Patriots tight end was already serving a sentence of life without parole for another murder in 2013.
Attorney William Kennedy represents the estate of Safiro Furtado and the family of Daniel de Abreu in wrongful-death lawsuits against Hernandez.
The jury in that case found Hernandez not guilty of first-degree murder but convicted him of unlawful possession of a gun, and the judge sentenced him to an additional four to five years in prison - separate from his existing life sentence.
Aaron Hernandez had the Bible verse "John 3:16" scrawled on his forehead when he was found dead in his prison cell Wednesday, reports say. He had attempted to block the door from the inside by jamming it with various items, prison officials said. His death came less than a week after his acquittal on murder charges in the shooting deaths of two men in Boston in 2012.
"The family and legal team is shocked and surprised at the news of Aaron's death", Baez said in a statement to the Boston Herald. [He] hanged himself utilizing a bedsheet that he attached to his cell window.
"It is a distinct honor for us to celebrate what was unequivocally our sweetest championship with a very good friend and somebody whose mental toughness and strength I greatly admire", said Kraft.
MA state police say they have launched an investigation into former National Football League star Aaron Hernandez's death by hanging in prison.
Hernandez's agent, Brian Murphy, tweeted that there was "absolutely no chance" Hernandez took his own life. He says Hernandez's loved ones are "heartbroken and determined to find the truth surrounding his untimely death". Prosecutors claimed Hernandez opened fire at Abreu and Furtado's vehicle while it was stopped at a red light in Boston because he was angry that one of them spilled a drink on him at a club.
"It's an unusual circumstance, and we would have to prepare carefully before we decide to go about it", said Thompson, Hernandez's lawyer.
In his three years with the Patriots, Hernandez scored 20 touchdowns and appeared in Super Bowl XLVI in 2012. The team released him in 2013 after he was arrested for killing Lloyd.