 -- The death of former NFL star Aaron Hernandez has been ruled a suicide, the Worcester County District Attorney's office said in a statement Thursday.

Chief medical examiner Dr. Henry N. Nields concluded that the cause of death was asphyxia by hanging. An investigation into Hernandez's death by the Massachusetts State Police found "cardboard jammed into the door tracks" of Hernandez's cell to "impede entry," officials said. Investigators determined that Hernandez was alone at the time of his death and said there was no signs that anyone had struggled to enter his single-inmate cell. The first person to enter was a correction officer at 3:03 a.m., according to officials.

Investigators found three hand-written notes next to a Bible in his cell, officials said.

Hernandez's body has been released to his family for burial, but the medical examiner has retained custody of his brain despite the family's wish to donate it to the Boston University CTE Center, Hernandez's defense attorney, Jose Baez, said. CTE stands for Chronic Traumatic Encephalopathy, a type of brain injury found in athletes and others who have experienced repetitive brain trauma.

Dan Bennett, Massachusetts' secretary for public safety and security, said that Hernandez's brain would be released to researchers soon.

“The Office of the Chief Medical Examiner is conducting an investigation into the circumstances of Aaron Hernandez’s death, which may require further analysis of his body. Once that is complete, the brain will be released to Boston University. No one is going to stand in the way of the family’s wishes for Boston University to have Aaron Hernandez’s brain,” Bennett said in a statement.

Massachusetts Gov. Charlie Baker said on Thursday that he has confidence in the investigation being conducted by the state's department of correction. Hernandez was found dead in his cell on Wednesday.

“Look, any time anybody kills themselves in a prison, something clearly went wrong,” the governor said, adding that he supports the department's leadership.

Baker said that at this time, he does not believe anyone has been disciplined in the wake of Hernandez's death.

Hernandez was found hanging from a bedsheet tied to the window of his cell at the Souza-Baranowski Correctional Center in Shirley, Massachusetts.

One of Hernandez's attorneys, Ronald Sullivan, told the Boston Globe that Hernandez was on the phone with his longtime fiancee, Shayanna Jenkins-Hernandez, about seven hours before his death. Sullivan did not say what the couple discussed but noted they were on the phone until about 8 p.m.

The Massachusetts State Police and Worcester County District Attorney's Office have launched an investigation into Hernandez's death. Police and prosecutors will look at Hernandez's recent time in prison, including prior fights and any gang-related disputes that might have occurred.

His legal team will also be conducting its own investigation, Baez said.

Hernandez was dropped from the New England Patriots' roster in 2013 after he was arrested for the murder of his friend, Odin Lloyd.

This is a developing story. Please check back for updates.