Mollie disappeared after going for a run on the evening of July 18, and was reported missing after she failed to show up for work the next day.
Authorities say a body was found in a farm field in the early morning hours of August 21, with Rivera acting as a guide for investigators. The body, believed to be Mollie, was covered by corn stalks. A preliminary autopsy report released Aug. 23 found that she died from "multiple sharp force injuries."
What Mollie's suspected killer claims happened
Authorities first became aware of Rivera after he was seen in surveillance video turned over to police by a neighbor. Investigators say the video shows Rivera following Mollie before she disappeared.
After his arrest, Rivera allegedly told investigators he approached Mollie, running behind and alongside her.
The suspect says he remembers her pulling out her phone, telling him she would call police if he did not leave her alone.
Then Rivera claims to have "blocked" his memory. He told investigators this is something he does when he is upset, KCRG reports.
When he came to, Rivera alleges he was in his car with headphones in his lap. That's when he said he knew there was a woman in the trunk. He said he looked in the trunk and saw that Tibbetts had blood on the side of her head. He carried her to a cornfield, where he left her.
Rivera identified as an undocumented immigrant
The U.S. Department of Homeland Security says Rivera is undocumented and has been living in the United States between 4 and 7 years.
Rivera's employer, Yarrabee Farms, said it had no idea Rivera was an undocumented immigrant because he provided them with a different name when he was hired and vetted. He had worked for the farm for four years and "was an employee in good standing," the company said in a statement.
A representative from the company told the Associated Press that they were shocked.
Iowa officials say Rivera is being held on a federal immigration detainer. It's unclear whether Rivera has ever been suspect to deportation proceedings, but he has no criminal history in Iowa, according to AP.
In court documents submitted Wednesday, Rivera's lawyer said that he is here legally and asked that the government not be allowed to say he is here illegally. The U.S. Citizenship and Immigration Services responded with a statement that there is "no record" Rivera is here legally.
The suspect also told investigators he had seen Mollie previously, but it is still unclear whether there was any connection between Rivera and the victim.
His girlfriend, Iris Monnarez, was friends with Tibbetts on Facebook. Investigators maintain that it's unclear whether they knew each other. They said it's common for those in a small town to be friends on Facebook if they went to the same high school, as Tibbetts and Monnarez did, even without being actually acquainted.
Rivera has a young daughter, according to the Associated Press. He lived in a trailer owned by his employer.
Rivera, who has been working for family-owned Yarrabee Farms since 2014, was praised as an employee, according to his lawyer.
"He showed up every day and he did his job. He was patted on his back. They turned a blind eye to the reality of documentation," defense attorney Allan Richards said.
Manager Dane Lang said "nobody saw a difference" in his demeanor at work after July 18.
First court hearing
Rivera did not enter a plea during a court appearance Wednesday. His cash-only bond was set at $5 million.
His next hearing is set for August 31.
The Associated Press contributed to this report.
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