Cristhian Rivera (center)
Charlie Neibergall/AP/Shutterstock
August 23, 2018 06:33 PM

The 24-year-old accused of killing Iowa college student Mollie Tibbetts — who had been missing for a month until her body was discovered on Tuesday — had no criminal record before he was charged this week with first-degree murder.

And so, among the mysteries still swirling in the case, is the nagging question: What may have motivated someone like that to do something like this?

“All of a sudden he just does it,” says Mitch Mortvedt, assistant director of the Iowa Division of Criminal Investigation, referring to suspect Cristhian Rivera. “Who escalates to abduction, kidnapping and homicide that fast?”

“That is part of the ongoing investigation,” Mortvedt tells PEOPLE. “We want to dig into his background — and not just the last four weeks. Who is this guy? Where has he been? What has he done? How can we verify that?”

Mortvedt says Rivera was not on the “radar” of investigators for much of the nearly five weeks they searched for Tibbetts after she was reported missing on July 19.

Tibbetts, a 20-year-old psychology student at the University of Iowa, was last seen alive the night of July 18, on her regular evening run around her hometown of Brooklyn, Iowa.

Mollie Tibbetts

According to authorities, a crucial break in the case came in recent days when Rivera was linked to a black Chevy Malibu seen driving back and forth in the same area where Tibbetts was jogging.

Questioned on Monday, Rivera allegedly admitted to crossing paths with Tibbetts and finding her dead in his trunk — but he “blocked” what happened in the time between, according to an arrest warrant affidavit.

Police say he told them that he dragged and then carried Tibbetts’ body into a cornfield about 11 miles away from where she was taken and covered it with plants.

He allegedly led law enforcement back to her remains on Tuesday morning. An autopsy completed the next day showed she died from “sharp force injuries,” but additional details have not been released.

RELATED VIDEO: Mollie Tibbetts Suspect Cristhian Rivera Appears in Court

Rivera “seemed to be drawn to her on that particular day and chose to abduct her,”  the Iowa DCI’s Rick Rahn told reporters on Tuesday.

Rahn said Rivera allegedly admitted seeing Tibbetts previously, but he did not elaborate. Authorities have not revealed when, exactly, Tibbetts was killed.

Mortvedt tells PEOPLE that he can’t discuss whether or not she was held for any period of time after being taken. He says he isn’t sure if Rivera may have spotted Tibbetts previously while she was out running or while they were “out and about” in the small community.

A motive has not been confirmed.

RELATED: A Timeline of the Mollie Tibbetts Case Since She Disappeared

Rivera made his initial court appearance on Wednesday and is set to return on Aug. 31. He remains in custody in lieu of $5 million bond.

He has not entered a plea but his attorney, Allan Richards, has reiterated that he is innocent until proven guilty and that no evidence has yet been presented in court.

“He was just part of the community, an all-American boy working really hard,” Richards tells PEOPLE of Rivera. “Whether or not there was something that happened that was very outrageous, that is a different matter. I have seen no evidence on that.”

Mollie Tibbetts
Poweshiek County Sheriff's Office

Iowa authorities said Rivera is an undocumented immigrant who lived in the area for four to seven years.

Richards says he is originally from Mexico and worked for several years in the local dairy industry after immigrating at 17.

“What if it was somebody else? What if the police browbeat this guy to give him whatever confession they say, which is yet to be seen?” Richards argues. “Why not give him the presumption of innocence, at least in the first few days of the charges? I am withholding judgment until something comes up otherwise.”

RELATED: Mollie Tibbetts’ Dad ‘Reluctantly’ Left Search for Her in Iowa Days Before Her Body Was Found

Tibbetts’ family issued a brief statement after her body was found. They did not mention her accused killer, instead saying, in part: “We thank all of those from around the world who have sent their thoughts and prayers for our girl.

“We know that many of you will join us as we continue to carry Mollie in our hearts forever.”

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