Rivera went by the alias "John Budd" around Yarrabee Farms, according to sources who spoke with ABC affiliate KCRG on the condition of anonymity. The employer declined to confirm or deny this.
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RELATED: What happened to Mollie Tibbetts? An overview of the case
Tibbetts, a 20-year-old college student from Iowa, went missing while on a jog on July 18. Her body was found on August 21 after authorities said Rivera led them there.
Rivera, 24, said that he remembers pursuing Tibbetts in his car and speaking to her and that he remembers leaving her bloodied body in a cornfield, but that he "blocked out" his memory as to what happened in between, according to authorities.
RELATED: A detailed timeline of the Mollie Tibbetts case
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Rivera, who has waived his rights to a preliminary trial, has been charged with murder. If convicted, Rivera could face life in prison without the possibility of parole. There is no death penalty in Iowa.
There was some confusion about the immigration status of Rivera, who is from Mexico. Authorities have said there is no record that Rivera is in the country legally, but his lawyer initially stated that Rivera had come here legally as a minor. Rivera's family later hired new counsel, the Des Moines Register reports.
Agents from the Immigration and Customs Enforcement, the Iowa Division of Criminal Investigation, and the county sheriff's office came to the farm on Thursday morning and appeared to be looking around and talking to workers for about two hours, KCRG reports.
Rivera began working at Yarrabee Farms in 2014 and was living on the property. Rivera's employers said they were shocked to find out he was the suspect in the case.