PRELIMINARY autopsy findings show that Iowa college student Mollie Tibbetts was killed by multiple “sharp force injuries”.

The Iowa Medical Examiner’s Office ruled the manner of death was homicide, according to a statement released this morning by the Iowa Division of Criminal Investigation.

The autopsy also confirmed that the remains recovered on Tuesday were that of Tibbetts.

It came as her accused killer was reportedly working at a dairy farm under a completely different name, according to his employer.

Cristhian Rivera, 24, used another person’s out-of-state ID card and Social Security number to obtain the job at Yarrabee Farms in Poweshiek County, Iowa, his boss said.

“This employee was considered in good standing,” explained Dane Lang, the farm’s manager at a press conference.

“What we learned within the last 24 hours is that our employee is not who he said he was. This was shocking to us.”

According to the New York Post, Lang told reporters on Wednesday that Rivera worked at Yarrabee for roughly four years, but was never run through the federal E-Verify program. The farm is owned by several generations of the Lang family, including prominent Iowa Republican Craig Lang.

Surveillance video was used to track down Mr Rivera, whose car was in footage that showed Ms Tibbetts jogging in a rural area near her hometown of Brooklyn, Iowa, around 7.30pm on July 18.

Mr Rivera told investigators he became angry when Ms Tibbetts showed a mobile phone and threatened to call police.

He said he panicked and then “blocked” his memory.

Mr Rivera claimed he did not recall what happened but found an earpiece from headphones in his lap and realised he’d put Tibbetts’ in the boot of his car.

He opened the boot and noticed blood on the side of her head.

Rivera was checked out via the Social Security Administration’s verification service, which can be easily beaten with another person’s state ID and Social Security card.

“There is rampant fraud,” said Bill Riley, a former US Immigration and Customs Enforcement agent who now works at the consulting firm Guidepost Solutions.

“Even experts like myself, we can say with fairly reasonable certainty — but not 100 per cent — whether a document is fake or not,” he told the Associated Press.

While authorities say Rivera is in the country illegally, his lawyer, Allan Richards, insisted on Wednesday that his client “has the legal documents” proving he is not.

Mr Richards acknowledged on Wednesday that his client received his paycheck under a different name and that he was uncertain of his immigration status. He said he was prepared to argue that his client was in the country legally, noting that he came to the US as a minor and had worked and paid taxes for years.

“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,” Mr Richards said.

Mr Richards also lashed out at US President Donald Trump for publicly declaring his client guilty within hours of his arrest on Tuesday.

“Sad and Sorry Trump has weighed in on this matter in national media which will poison the entire possible pool of jury members,” Mr Richards wrote in a court filing.

Mr Trump has claimed that people living in the US illegally often commit crimes, and has prioritised cracking down on illegal immigration.

Mr Trump called Tibbetts a “beautiful young girl”. “She was killed by a horrible person that came in from Mexico, illegally here,” the US President said.

However, Tibbetts’ aunt also spoke out to say “evil comes in every colour”, as her family plead for her death not to become a political tool.

US Citizenship and Immigration Services has no record of Rivera, who is charged with first-degree murder, having “any lawful immigration status” whatsoever.

“We continue to co-operate with the investigation, and we have provided information about our former employee, including his hiring records, to authorities,” Craig Lang said during a press conference on Wednesday. “There will be plenty of time later to discuss immigration. However, now is not that time.”

Yarrabee Farms is located less than five kilometres from where Tibbetts, 20, was last seen jogging the night she was abducted.

Some are blaming the Lang family for her death, saying they could’ve got Rivera deported back to Mexico had they just done their due diligence.

“Yarrabee Farms owes the Tibbetts family an explanation of why it did not use E-Verify,” said RJ Hauman, government relations director at the Federation for American Immigration Reform.

Dane Lang told reporters that Rivera seemed fine in the weeks before Tibbetts’ disappearance and wasn’t acting strange at all.

“I wish I had seen a difference, but I didn’t,” he said.

The Lang family has received countless death threats in the wake of Rivera’s arrest, including threats against their dog.

“This is a really scary situation,” Dane Lang said. “And the unfortunate thing is, people are paying attention to me and we’re forgetting about the heartbroken family in Brooklyn and the heartbroken community.”

A judge on Wednesday agreed to increase Rivera’s bail to $US5 million ($A7 million) from $US1 million ($A1.3 million) after prosecutor Scott Brown noted he was a potential flight risk who was charged with a “heinous crime”.