Rivera led authorities to a corn field on Tuesday morning, where they found a body covered in corn leaves.
The state medical examiner has determined that the 20-year-old's death was a homicide, but no details were released about what caused the sharp force injuries.
Additional details of the autopsy are not available at this time.
Investigators have not said how Tibbetts was killed or what happened in the moments after she disappeared July 18 while jogging. A massive five week search ended on August 21 when investigators announced the arrest of 24-year-old Cristhian Rivera, who worked at a nearby dairy farm, and the discovery of Tibbetts body.
The lawyer for the man charged with abducting and murdering Mollie Tibbetts said the suspect is not an illegal immigrant, contradicting what authorities said Tuesday, according to the Des Moines Register.
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In a motion, Richards said Rivera was authorized to work in the United States, but the employee issued a statement later that day saying the defendant gave them false information to work for them, according to USA Today.
"The loss of Mollie Tibbetts is a devastating reminder that we must urgently fix our broken immigration laws", a White House tweet said Tuesday. "We're getting it changed but we have to get more Republicans".
Smith said also she supports comprehensive immigration reform, including border security and a path to citizenship for people who are in the country working and paying taxes. An undocumented immigrant, Jose Ines Garcia Zarate, was later convicted on gun charges related to Steinle's death.
Tibbetts was studying psychology at the University of Iowa and wanted to get a doctorate and write books, her father said.
However Lang's son, Dane, told reporters on Wednesday that Rivera had used another name when applying for work there four years ago and that managers did not use the E-Verify system.
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During a news conference Tuesday, authorities said Rivera was employed and lived in the area for four to seven years, but they would not disclose his employer. He's being held on a $5 million cash-only bond. Rivera said he chased her and blacked out. His defense attorney, Allan Richards, said Rivera came to the USA as a minor, held a job in the community and has no prior criminal record.
A U.S. Immigration and Customs Enforcement representative said that the agency lodged an immigration detainer on Rivera after his arrest, so if he is ever released, he would be deported to Mexico.
Rivera's Facebook page described him as being from Guayabillo, a community of less than 500 people in the Mexican state of Guerrero.
Rivera, 24, said he dragged, then carried Tibbetts' body 60 feet into an isolated cornfield. It's about a three-hour drive from the resort city of Acapulco. She pulled out her phone and told him, "I'm gonna call police".
A conviction on first-degree murder carries a mandatory sentence of life in prison without parole in Iowa, which does not have the death penalty.
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