Day was 25 years old when he went missing Aug. 24. He was studying at ISU in Bloomington to be a doctor.
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His car was found two days later an hour north in the town of Peru. A week after that, his body was found floating in the Illinois River in LaSalle County, according to officials.
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The LaSalle County coroner said a forensic autopsy did not reveal any evidence of injury before Day's death, "such as manual strangulation, an assault or altercation, sharp, blunt or gunshot injury, infection, tumor, natural disease, congenital abnormality, or significant drug intoxication."
It is still not known how Day ended up in the Illinois River.
At ISU they continue to push investigators to find answers for Day's death. His mother issued an emotional plea to his classmates Monday night.
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"I did not send Jelani here so I'd have to be here in front of you all today and I can't have my son no more," said Carmen Bolden Day. "Jelani was on this campus for a week and a half."
She also told the ABC7 I-Team that Day was a strong swimmer and a member of his high school swim team for a period of time. She vehemently believes someone killed her son.
Reverend Jesse Jackson also spoke at the ISU Black Student Union, saying he stands with Day and is also urging students to keep seeking answers.
"I need justice for Jelani. Because, like Reverend Jackson said, somebody who did this, they know. And they are walking around," Day said.
Day's mother said his personal items were found in different areas and she wants to know how her son ended up in the river.
RELATED: Jelani Day case: Mystery grows around disappearance, death of ISU graduate student
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Day's mother also said his phone has not been found, and is asking Apple to release phone records that may help investigators.
So far, investigators have only called the circumstances surrounding the disappearance "unusual."
Police are still investigating the case, along with the FBI.
His funeral was held last week.