Her business is one of the largest in Brooklyn, the central Iowa town where Tibbetts disappeared while out for a run on July 18.
Investigators have not yet confirmed the identity of the remains, but they believe them to be those of Tibbetts.
Yarrabee Farms, owned by a well-known family in Iowa ag, initially said they had gone through the proper legal channels to hire Rivera.
"It seemed that he followed her, seemed to be drawn to her on that particular day, and for whatever reason, he chose to abduct her", said Rick Rahn, a special agent in charge with the Iowa Division of Criminal Investigation.
"I'm sure she's as shocked as we are".
Rivera alleged he blacked out at some point and woke up at an intersection in rural Poweshiek County, court documents state.
"I've voted twice to build a stronger physical and electronic security system along our border, a nationwide implementation and enforcement of a strong E-Verify system, stop catch and release, add immigration judges, and work towards a more merit-based immigration system", he said in a news release. Rivera said that while he "blocked out" memory of the incident, he did recall dumping her body.
Rivera dragged the woman to an isolated area in the cornfield, he told police, then covered her body with corn leaves and drove away.
A document filed by Rivera's attorney Wednesday calls into question Rivera's immigration status. In interviews with reporters and in court filings, attorney Allan Richards said his client is in the United States lawfully.
Iowa Republican Governor Kim Reynolds echoed Trump's approach, blaming Tibbetts' death on the nation's immigration laws.
"We have found no record in our systems indicating he has any lawful immigration status", Bars said.
Another Trump loyalist granted immunity as prosecutors close in on Trump business
These individuals may spell the beginning of the end of the Trump presidency, and seem to have Trump remaining silent - for now. He admitted he violated campaign finance law by helping provide Trump's campaign excessive and illegal contributions.
Neudauer said ICE issued an immigration detainer request Tuesday.
Even in a community where "everybody knows everybody", Rahn said that Rivera kept to himself and that investigators are looking more deeply into his background. They said the 24-year-old worked under a different name and was a good employee who helped take care of their cows and got along with co-workers.
Georgia's immigration law led to more than $140 million in unharvested crops in 2011 because so many workers fled the state, according to a report commissioned by the Georgia Fruit and Vegetable Growers Association. He faces life in prison without the possibility of parole if convicted.
Rivera said he then drove to a field entrance and into a cornfield driveway, before realizing he had Tibbetts in the trunk, the affidavit said.
"In today's tight economy, labor is scarce, and finding employees for open positions can be hard", the association said in a statement.
"We get to know the family". "The best gift we can give is to be a little like Mollie today". I have daughters. I want them safe. "All of us are saddened by the tragic death of Mollie and the realization that one of our co-workers was involved".
President Donald Trump has repeatedly used the killing of 20-year-old college student Mollie Tibbetts as fuel for a new round of political attacks less than three months out from the midterm elections. On a visit to Des Moines last week, Vice President Pence met with the Tibbetts family, telling them "you're on the hearts of every American".
The illegal alien raped a very young child. "But what happened to Mollie was a disgrace, and our hearts go out".
"As Iowans, we are heartbroken, and we are angry", Reynolds tweeted on Tuesday, soon after Rivera's arrest. Iowa's Republican governor, facing a tough re-election challenge in November, blasted an immigration system that "allowed a predator like this to live in our community".
Joe Gruters, a Florida state legislator and a co-chairman of Trump's re-election campaign in Florida posted on Twitter simply: "Time to build the wall".
"Let's let the process go", he said. On behalf of Mollie's entire family, we thank all of those from around the world who have sent their thoughts and prayers for our girl.