His family said they suspect foul play as the mystery surrounding his death grows.
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"I'm very upset because I can't even look at my son's body. I can't say a proper goodbye to him," said his mother, Carmen Borden Day.
Day, who was studying to be a doctor, was living and going to school in Bloomington, Illinois. He was last seen on campus on Aug. 24 and 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, according to officials.
Peru police investigators are now working to identify a man captured on security cameras knocking on at home not far from where Day's remains were located. The video was taken the day after the missing student was last seen.
Police working to identify man seen on surveillance video knocking on door in LaSalle
The FBI is now working with local investigators as they try to figure out if the unknown subject is connected to Day's case.
The Day family's attorney said loved ones suspect foul play.
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"The wallet was found in a different area. The lanyard, which had a school ID on it, was not found with the wallet -- it was found in a different area," said attorney Hallie Bezner.
RELATED: Jelani Day's family looking for answers after missing ISU student found dead in river
Authorities, so far, are only calling the circumstances surrounding the disappearance "unusual."
"The way his vehicle was, you know, it was off of a parking lot," said Peru police. "That's very unusual to just find a car like that."
Investigators are now working to retrace Day's final movements.
"Jelani did not just disappear into thin air," his mother said. "Somebody knows something; somebody's seen something, and I need somebody to say something."
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The 25-year-old's death has garnered the attention of Viola Davis, Lizzo and other celebrities.
At this point, authorities have not named any suspects in this case.
The community will hold a special memorial for Day on Oct. 9.
We will continue to share updates on Jelani's case as information becomes available.