"Mr. Hernandez hanged himself utilizing a bedsheet that he attached to his cell window".
Murphy Tweeted that there was "absolutely no chance he took his own life".
The attorney called on authorities to "conduct a transparent and thorough investigation" and promised his own examination of the death.
In a maximum-security prison outside Boston, about an hour from the stadium where he played alongside stars such as Tom Brady and Rob Gronkowski, Hernandez jammed the door to his one-man cell lest guards try to stop him and put an early end to the life-without-parole sentence he received for a 2013 murder.
"Lifesaving techniques were attempted on Mr. Hernandez and he was transported to UMASS Leominster, where he was pronounced dead at 4:07 a.m.by a physician at the hospital". He says Hernandez's loved ones are "heartbroken and determined to find the truth surrounding his untimely death".
Under Massachusetts law, defense attorneys can seek to have convictions vacated when a defendant dies before an appeal is heard. He had also recently been on trial for the double murder of Daniel de Abreu and Safiro Furtado outside a Boston nightclub in 2012, but was found not guilty this past Friday.
Massachusetts State Police are now investigating Hernandez's death.
While serving the life sentence in a MA prison, double homicide charges were also brought against him for the 2012 murders of Daniel Abreu, 29, and Safiro Furtado, 28.
Massachusetts State Police has now launched an investigation into the suicide at the Souza Baranowski Correctional Centre in the town of Shirley.
But the team released him in 2013 after he was arrested in the killing of Odin Lloyd, a semi-professional football player who was dating the sister of Hernandez's fiancee.
At the height of his career with the New England Patriots, now Super Bowl champions, Hernandez was on a five-year contract worth $40m (£31m). "I knew we had raised doubt throughout that case". He was just 27 years old.
He was alone in his cell and "attempted to block his door from the inside by jamming the door with various items".