11:01 AM PT -- Fuller's brother called 911 after the RB collapsed to the ground and told the operator it appeared he was having a seizure.

The caller said Fuller had "passed out" -- though it's unclear if he was conscious at that point.

C.J.'s brother and other family members desperately tried to help Fuller -- following the operator's instructions -- and begged for an ambulance to be sent over quickly.

8:20 AM PT -- A source extremely close to Fuller tells us the football star had gone to physical therapy to rehab his knee in the hours before his death ... and it was clear something was wrong.

We're told Fuller tore his ACL playing in a flag football game roughly 2 weeks ago and had surgery to repair the injury at the end of September.

Fuller went to physical therapy for the first time on Wednesday (10/3) as part of his recovery and completed a full session without issue.

When he arrived back to his home, we're told he "went to the ground" and asked for water -- clearly feeling something was wrong.

People close to C.J. called for an ambulance and he was rushed to a nearby hospital. He was pronounced dead a short time later.

There are rumblings Fuller's death was a direct result of complications from the knee surgery -- but we're told that hasn't been determined yet.

We spoke with the coroner who told us a death investigation is underway.

7:58 AM PT -- We spoke with the Pickens County coroner who tells us there is death investigation underway but she notes, "Nothing suspicious."

Ex-Clemson running back C.J. Fuller -- a key player on Clemson's 2016 National Championship team -- has died, the school has confirmed.

He was only 22 years old.

The cause of Fuller's death is unclear. We're working on gathering information.

Fuller played in 43 games for the Tigers -- and proved to be a talented and capable back on the field. Off the field, Fuller had issues ... he was arrested back in March for allegedly robbing someone at gunpoint in South Carolina.

Clemon's coach Dabo Swinney issued a statement saying, "Our thoughts, prayers and deepest sympathies are with C.J.'s family."

"I've known C.J. a long time and watched him grow up through the Easley rec leagues [in South Carolina] all the way through Easley High School. I'm proud of what he accomplished as a Clemson Tiger, most of all his accomplishment of being a Clemson graduate."

"Our deepest condolences and the thoughts of our program are with his family this evening. May he rest in peace."

Fuller's former Clemson teammate Shaq Lawson (who now plays for the Buffalo Bills) also commented on social media ... saying, "My brother I am hurting right now."'

"Somebody wake me up from this nightmare. My brother was supposed to come see me tomorrow. I love you bruh until we meet again."