Murdock's body was found in his car parked by Middleton High School in Tampa, Florida, where he was a high school prodigy.
"It does appear to be a suicide, but we have to investigate all angles," Tampa Police spokeswoman Andrea Davis said.
Davis said reports that Murdock had shot himself in the chest - like some other former NFL players to supposedly preserve their brains for the study of head injuries - were incorrect.
"It was a gunshot wound to the head," she said after meeting again with the homicide sergeant.
The Titans said Murdock had not reported to training camp over the weekend because of an unspecified personal issue.
"We are shocked and saddened to hear of O.J. Murdock's death this morning," the Titans said in a statement.
Titans running back Chris Johnson was among those to react on Twitter to the news: "RIP to OJ murdock was a good guy and teammate. Cherish Every moment of life cause u can be here today an gone tomorrow," he said.
Rated among the top high school receivers in the US in 2005, Murdock began his career at the University of South Carolina. He was suspended after a Florida shoplifting charge the following year.
He ended his college days as a standout at Division II Fort Hays State in Kansas. In 2011 he was signed by the Titans after appearing in the professional scouting combine.
He sat out the season on injured reserve after suffering an Achilles injury in training camp.
"In his brief time here, a number of our players, coaches and staff had grown close to O.J., and this is a difficult time for them," the Titans said on the team website.
"He spent the last year battling back from an Achilles injury as he prepared for this year's training camp. Our thoughts and prayers are with his family and friends as they try to cope with this tragedy."
Murdock, 25, never appeared in a National Football League game for the Titans.
