The evidence against the man accused of murdering 20-year-old Mollie Tibbetts in Iowa is overwhelming -- even if parts of his police confession are ruled inadmissible in court, a law enforcement source close to the case told Fox News on Tuesday.

Defense attorneys for Cristhian Bahena Rivera -- who has been charged with first-degree murder in Tibbetts' death -- argued that his rights were violated during his initial interview with investigators. The state has since indicated that some of Rivera's statements cannot be used at trial because his Miranda rights initially were not read to him in their entirety.

A judge is set to decide next month what evidence will be presented to jurors at the February trial of Rivera, 25, an illegal immigrant from Mexico accused of killing Tibbetts on July 18, 2018, after following the college student on her evening jog through the streets of her hometown of Brooklyn, Iowa.

Whatever the outcome, the law enforcement source told Fox News there's an abundance of other damning evidence incriminating Rivera in Tibbetts' death. The statement echoed claims from the state in its most recent court filing. The source spoke on condition of anonymity because the person was not authorized to speak publicly about the investigation. The source did not elaborate on the evidence investigators had, but it has been reported that Tibbetts' blood was found in the trunk of Rivera's car.

MURDER SUSPECT WORKED LESS THAN THREE MILES FROM WHERE MOLLIE TIBBETTS WAS STAYING

According to court documents, Rivera, who speaks some English, was interviewed by police for a total of eight hours and 30 minutes on Aug. 20, 2018. At approximately 11:30 p.m., several hours into the interview, Rivera was given his first Miranda warning by the lead interviewer, Iowa City Police Officer Pamela Romero, whose native language is Spanish. That warning was "incomplete in that the officer giving it [Romero] inadvertently omitted informing the defendant [that] what he says can be used against him in court at a later time," according to a document filed by the prosecution last week.

Mollie Tibbetts, seen in an undated photo provided by her family, was killed in 2018. (Rob Tibbetts)

When Rivera eventually led investigators to Tibbetts' remains in a cornfield, Romero read the suspect his Miranda Rights in full inside a vehicle at approximately 5:50 a.m. on Aug. 21. After the second Mirana warning, "the defendant knowingly waived his Miranda rights and continued to speak with officers," the document read.

"Following his second warning, the defendant made numerous statements that implicate him in Mollie Tibbetts' murder," it continued.

Tibbetts' disappearance shook the small town of Brooklyn. For weeks, local and federal investigators, assisted by townspeople who volunteered to comb ditches and fields, scoured cornfields and ponds for any trace of her.

MOLLIE TIBBETTS DISAPPEARANCE A TOTAL SHOCK, RESIDENTS SAY

Tibbetts was dog-sitting for her longtime boyfriend, Dalton Jack, and his brother the night she disappeared. The Jack brothers were working about a hundred miles away at a construction site on the night of her disappearance.

From the onset of the investigation, detectives focused on forensic electronic evidence to find the University of Iowa student. Many were convinced her Snapchat or Instagram activities would help them find her – dead or alive. However, surveillance video and traditional police work eventually lead them to Rivera.

The suspect worked at a dairy farm less than three miles from where Tibbetts was staying on the night of her disappearance. In the aftermath of Rivera leading investigators to Tibbetts’ body, Yarrabee Farms was scrutinized for its employment practices.

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Dane Lang, one of the owners of the farm, said the company followed federal employment laws and Rivera passed the government’s E-Verify system – although the veracity of that system has been scrutinized in the past.

Rivera told investigators he was following Tibbetts on her run and, according to an affidavit, panicked when she threatened to call police and “blocked” his memory. Then, he said he pulled into the entrance of a cornfield and found Tibbetts, with the side of her head bloodied, in the trunk of his Chevy Malibu.

Fox News' Matt Finn in Iowa City, Iowa, contributed to this report.