Former NFL star Aaron Hernandez was jailed in 2013
The former New England Patriots tight end was jailed in 2013 for a killing.
However, he was recently acquitted of murder.
Last week, he was found not guilty of killing Daniel de Abreu and Safiro Furtado after he was accused of opening fire on a car.
Mr Hernandez was then convicted of a single charge: unlawful possession of a gun.
Aaron Hernandez, once of the New England Patriots, has been found dead
He was set to spend another four to five years in jail for the conviction.
The 27-year-old was found by officers at the Souza Baranowski Correctional Center in Shirley, Massachusetts on Wednesday morning.
Aaron Hernandez #81 of the New England Patriots
Aaron Hernandez was discovered hanged in his cell by corrections officers at the Souza Baranowski Correctional Center in Shirley
Massachusetts Department of Corrections
A statement from the Massachusetts Department of Corrections said: "On April 19, 2017 Aaron Hernandez was discovered hanged in his cell by corrections officers at the Souza Baranowski Correctional Center in Shirley, Massachusetts at approximately 3:05am.
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"Lifesaving techniques were attempted on Mr. Hernandez and he was transported to UMASS Leoinster where he was pronounced dead at 4:07am by a physician at the hospital.
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"Mr Hernandez was in a single cell in general population housing unit. Mr Hernandez hanged himself utilizing a bed sheet that he attached to his cell window. Mr Hernandez also attempted to block his door from the inside by jamming the door with various items.
"The Massachusetts State Police are on scene and the investigation continues. Mr Hernandez's next of kin have been notified."