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(CNN) —  

More than a month ago, Mollie Tibbetts went for a jog and never returned.

On the evening of July 18, as the 20-year-old college student was out running, she encountered a man in a black Chevy Malibu who got out of his car and started jogging with her, officials say.

The suspect, Cristhian Bahena Rivera, 24, told police he saw Tibbetts running and “pursued her in his vehicle” while she was running in an area east of Brooklyn, according to the document.

On Tuesday, Rivera was charged with first-degree murder in the death of Tibbetts. A body that officials believe is that of the Iowa college student was found on the same day.

“He actually tells us that he ran alongside of her or behind her,” said Rick Rahn, special agent in charge at the Iowa Division of Criminal Investigation during a news conference on Tuesday. “And then, at one point, he tells us that Mollie grabbed ahold of her phone and said, ‘You need to leave me alone. I’m going to call the police.’ And then, she took off running and he, in turn, chased her down.”

Rivera told investigators he panicked, became angry and doesn’t remember what happened next, according to the affidavit.

“He tells us that at some point in time, he blacks out,” Rahn told reporters.

Rivera told officers that he “‘blocked’ his ‘memory’ which is what he does when he gets very upset, and doesn’t remember anything after that until he came to at an intersection,” according to the affidavit.

He then drove to a driveway to a cornfield and said he noticed an earpiece from headphones on his lap, according to the affidavit. It was then he realized he had put the woman in the trunk of his car and when he went to the trunk, he “noticed blood on the side of her head,” according to the affidavit.

He told police he dragged then carried Tibbetts to a secluded location and left her there, covering her in corn leaves, according to the affidavit. Rivera later led investigators to the location.

Rahn told reporters he could not release information about how Tibbetts was killed. And the motive also remained unclear.

“I can’t really speak to you about the motive,” Rahn said. “I can just tell you it seems that he followed her and seemed to be drawn to her on that particular day and for whatever reason he chose to abduct her.”

Rivera, an undocumented immigrant, is being held on $1 million cash bail. The US Immigration and Customs Enforcement sent a detainer request to local authorities for Rivera, who is from Mexico, according to an ICE spokesman.

CNN’s Steve Almasy, Sheena Jones and Dave Alsup contributed to this report.