Authorities in Iowa have arrested a suspect in the slaying of Mollie Tibbetts, the 20-year-old University of Iowa student who went missing in late July, police announced at a news conference Tuesday.

Cristhian Bahena Rivera, 24, is charged with first-degree murder in Poweshiek County, Rick Rahn of the Iowa Division of Criminal Investigation said.

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Authorities first approached Rivera on Monday, Rahn said.

More than four weeks after Tibbetts was reported missing, her remains were believed to have been found Tuesday morning, dumped in a cornfield in Guernsey, Iowa, about 15 minutes by car southeast from Brooklyn, Iowa, where Tibbetts was last seen alive, a police source told PEOPLE Tuesday.

Image zoom Cristhian Bahena Rivera Poweshiek County

Rahn said Tibbetts’ body was covered with corn stalks and that Rivera led police to her body.

Police allege Rivera confessed to circling around Tibbetts a few times with his car while she was running in Brooklyn, then he approached her and started running alongside her. Rahn said Rivera told authorities that at one point Tibbetts told him to leave her alone, threatening to call the police.

Subsequently, Rivera allegedly chased Tibbetts down and at some point said he blacked out, Rahn said.

Image zoom Mollie Tibbetts Poweshiek County Sheriff's Office

Rahn would not say how Tibbetts was killed and would not comment on a possible motive, saying only that Rivera “seemed to be drawn to her on that particular day and chose to abduct her.”

Rahn said Rivera admitted to seeing Tibbetts before, but said that “beyond that, I won’t be able to say anymore about his interactions with Ms. Tibbetts.”

Rivera is an undocumented immigrant, Rahn said. Rahn said he “kept to himself” and had lived in the area between four and seven years.

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Rahn said the break in the case was when they found security cameras showing Tibbetts jogging, which led them to identify Rivera’s vehicle, which Rahn described as a black Chevy Malibu. Rahn said authorities “were able to determine that he was one of the last ones to have seen Mollie running.”

Subsequently, authorities located Rivera Monday, who allegedly led them to the location of Tibbetts’ body.

Rivera “was very compliant,” said Rahn. “He was willing to talk to us. There was no struggle.”

Tibbetts was last seen on the evening of July 18, while out for her evening run around Brooklyn, though Rahn said it was not clear when she died.

Rivera is being held in the Poweshiek County Jail in lieu of a $1 million cash-only bond, reports local station WHO. It was not immediately clear if he has retained an attorney.

RELATED VIDEO: Missing Student Mollie Tibbetts’ Dead Body Found in Local Field More Than a Month After She Vanished: Source

Tibbets, a sophomore at the University of Iowa, was living in the small rural town and the night of her run was staying with her boyfriend, Dalton Jack, in his older brother’s house.

According to police, Tibbetts was caring for the Jacks’ dogs, as both were out of town.

An intense search for the psychology major ensued, involving hundreds of volunteers.