Rivera was employed and resided in Poweshiek County for four to seven years, Iowa Division of Criminal Investigation Special Agent in Charge Richard Rahn said at a press conference in Montezuma on the afternoon of August 21.
Cristhian Rivera, age 24, has been charged with first degree murder in the case.
Rivera told investigators he became angry when Tibbetts showed a cell phone and threatened to call police.
Watch live as authorities continue to hold a news conference regarding the missing Brooklyn 20-year-old Mollie Tibbetts.
An autopsy to determine when and how the woman died is planned for Wednesday.
Tibbetts was studying psychology at the University of Iowa and wanted to get a doctorate and write books, her father said.
The Department of Homeland Security said Rivera has been in the country "between 4 and 7 years".
Greg Willey of Crime Stoppers of Central Iowa said he learned of the discovery from a close friend of the Tibbetts on Tuesday morning.
A man from Mexico living in the US illegally has confessed to kidnapping college student Mollie Tibbetts while she was running in her small Iowa hometown, killing her and dumping her body in a cornfield, authorities said Tuesday.
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There, Rivera told police he took Tibbetts out of his trunk and "noticed blood on the side of her head". Rivera led law enforcement to the area he allegedly remembered, and that is where officials discovered the deceased female.
The Iowa Division of Criminal Investigation is holding a news conference at 5 p.m. local time at the Poweshiek County Sheriff's Office in Montezuma.
Before she went missing, Tibbetts' brother dropped her off at her boyfriend's house so she could dog-sit, HLN reported.
Tibbetts, a 20-year-old student at the University of Iowa, had been missing since July 18. She took off running and he in turn chased her down.
"We tend to be a bigger target", she said, talking about women who walk or run alone. "We just spoke to the family and I told them they raised a great daughter".
Her disappearance set off a massive investigation and weeks of fruitless searches. They focused much of their efforts in and around Brooklyn, searching fields, ponds and homes. Investigators said she was last seen on a routine jog through the streets of the city. It is significant because if we had stronger border security, it is possible that Mollie Tibbetts would be alive today.
Vice President Mike Pence, who met with Tibbetts's family last week during a visit to Des Moines, said he was "heartbroken" by the news.
The cause of death has not been released and police have not given the name of a suspect, but her family and boyfriend have been cleared of suspicion, per the WHO-DT, an NBC affiliate.
While authorities declined to discuss what may have motivated Rivera to approach Tibbetts, Rahn said he had seen her before but was drawn to her on that particular day.