Illinois State University student Jelani Day died of drowning, a coroner determined on Monday, according to CNN.
LaSalle County Coroner Richard Ploch noted that the manner of death in Day's case remains unknown.
Day, 25, was reported missing on Aug. 25 after missing several of his university classes. His body was found in a river on Sept. 4 and was identified later in the same month.
Several factors relating to the environment Day's remains were found in complicated the forensic exam, including scavenger animals in the river and decomposition. He determined, however, "there was no evidence of any (pre-death) injury" that could have contributed to Day's passing.
Day's mother, Carmen Bolden Day, said the autopsy report findings don't tell the full truth of what happened to her son.
"I don't care what their press releases say, MY BABY WAS KILLED," she wrote in a Tuesday Facebook post. She noted that Day was an avid swimmer and didn't have a history of mental health issues or suicidal ideations.
She added, "These police departments want an EASY way out."
Bolden Day has requested the FBI's involvement in her son's case.