Lawrence Phillips' biological mother -- and the people closest to him -- do not believe the ex-NFL star committed suicide in his prison cell ... with his next of kin telling TMZ Sports, "I know for a fact he wouldn't do that."
We spoke with Tonissa Murdock -- who was listed as Phillips' next of kin with Kern Valley State Prison due to their close relationship for more than 20 years.
Murdock says her family took in Lawrence during the '90s and he's been like a brother to her ever since. Tonissa says they were extremely close and last spoke with him around the Christmas holiday and he was very upbeat and positive at time, focusing on becoming a better person.
"I know him well enough to know he wouldn’t commit suicide."
Tonissa says she's the one who had to break the news to Lawrence's biological mother -- and says they both are demanding answers from prison officials.
"I asked the coroner for information about his body and she was very unhelpful," Tonissa says ... adding that she was told Lawrence was found with a note -- but officials wouldn't reveal what it said.
Tonissa also says she's upset that she was notified at 7:15 AM -- hours after he was found -- preventing her from being able to meet Phillips at the hospital before he was pronounced dead.
In the past few months, Tonissa says Phillips was determined to clear his name in the April death of his cellmate -- telling her he only acted in self-defense.
"He knew that he was innocent about his cellmate. He faced adversity his whole life and during all those times he said suicide was never an option."
As for the prison, officials say they will examine Lawrence's body on Friday -- but so far, have not released any details regarding how exactly the ex-NFL star died.