Day, an Illinois State University graduate student studying speech-language pathology, was last seen in Bloomington on August 24. His body was found more than an hour north of there in Peru, more than 60 miles from his campus.
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Day's body was found floating in the Illinois River. But before that, his car was found in a wooded area of Peru, its license plates removed. It was the one concrete clue police found in his disappearance and one that his mother can't stop thinking about.
"My kids don't come to Peru for nothing," said Carmen Bolden Day. "Never was raised anywhere near here. Don't know anyone here. Jelani would not have known to come and park his car in this wooded area."
READ MORE | Jelani Day's family looking for answers after missing student found dead in river
The distance between where Day's car was found and where his body was found is another puzzling part of the mystery, nearly two miles away.
His mother says none of it makes sense.
"I've had to grieve while working and trying to find answers," she said.
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Her search for answers leads back to the ISU campus. On Aug. 25, one of Day's professors reported he didn't show up to class, and he wasn't picking up his phone.
"I didn't get a text message back. Usually, that's our thing. He'll call me right back," Bolden Day recalled. "That's the moment I said, something is wrong."
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Investigators said Day was last seen at 7:20 a.m. the morning before, at the student center. The same day, at 9:12 a.m., he was spotted on surveillance video at the Beyond Hello cannabis dispensary in Bloomington. That's the last known image of him.
Police said detectives looked through electronic data and that was the final location they can find. Police said they can look into "anything that's connected to electricity or a cell phone tower."
WATCH: Complete ABC7 coverage of Jelani Day
The FBI said it's hoping to get more information from Jelani Day's close contacts to better understand the circumstances surrounding his death.
But how Jelani Day's car got from Bloomington 60 miles north to Peru remains a mystery.
A trail near where his car was located leads to a nearby road.
"I guarantee you, my son didn't drive himself here. Whoever brought Jelani here, they were very familiar with this area," his mother said.
Earlier this week the LaSalle County coroner named Day's cause of death a drowning because he was found in the river, and because there was no evidence of injury to the body. But his mother is adamant that anyone in the community who believes he did this to himself is wrong.
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"Number one," Bolden Day said, "Jelani knew how to swim. For them to imply that Jelani drove his car into this space, took it over there, parked it, removed his license plates, then walked to where his body was found a mile and a half away from here, then took off his shoes and socks and his shorts and then put himself in this water, into this water, it doesn't make sense."
Bolden Day said her son's license plates and his phone still haven't been found.
The family is demanding answers and justice.
RELATED: Jelani Day's Mother pleads for federal help investigating son's death
"I know that Jelani knows that no matter what he was doing, whenever he needed me I was there. So I know that he knows that I am going to be there and I'm going to find out what happened to him," she said.
The police departments investigating this case said they're working tirelessly to find out what happened to Day, and are dedicated to providing the family with answers. But his mother said she feels she has received little assistance from local and state officials and is initiating a third-party private independent autopsy.
She's also pleading with Illinois Attorney General Kwame Raoul and the FBI to take over this case and help her find out what happened to her son.