MONTEZUMA, Iowa — A man from Mexico living in the U.S. illegally has confessed to kidnapping college student Mollie Tibbetts while she was running in her small Iowa hometown, killing her and dumping her body in a rural field, authorities said Tuesday.

Cristhian Bahena Rivera, 24, was arrested and charged with first-degree murder in the death of Tibbetts, whose July 18 disappearance set off a massive search involving state and federal authorities.

Rivera led investigators early Tuesday to a body believed to be Tibbetts in a cornfield about 12 miles (19 kilometers) southeast of Brooklyn, Iowa, where Tibbetts was last seen running, Division of Criminal Investigation special agent Rick Rahn said.

"I can't speak about the motive. I can just tell you that it seemed that he followed her, seemed to be drawn to her on that particular day, for whatever reason he chose to abduct her," Rahn told reporters at a news conference outside the sheriff's office in Montezuma, where Rivera was being jailed.

Investigators said they zeroed in on Rivera after obtaining footage from surveillance cameras in Brooklyn. The footage showed a Chevy Malibu connected to Rivera that was driving back and forth as Tibbetts was running in the area, Rahn said.

An affidavit attached to the criminal complaint against Rivera alleged that he admitted to investigators he got out of his car and started running alongside Tibbetts.

Tibbetts grabbed her phone and said she was going to call the police. The affidavit says Rivera panicked and then said he blacked out. Rivera next remembers seeing her earphones on his lap, and taking her bloody body out of the trunk of his car, it said.

"The defendant further described during the interview that he dragged Tibbetts on foot from his vehicle to a secluded location in a cornfield," the affidavit said.

Investigators said they had earlier searched the area for Tibbetts but didn't find her, noting the body was covered by corn stalks.

Investigators believe that Rivera is from Mexico and had been living in the area from four to seven years, Rahn said. He said that investigators have interrogated him with the help of a translator.

Rahn said an autopsy would be performed on the body Wednesday by the state medical examiner's office. He said that the autopsy would assist investigators in understanding whether she had been assaulted or tried to fight him off.

A conviction on first-degree murder carries a mandatory sentence of life in prison without parole in Iowa, whidh doesn't have the death penalty.

Tibbetts' disappearance set off a massive search involving dozens of officers from the FBI, as well as state and local agencies. They focused much of their efforts in and around Brooklyn, searching farm fields, ponds and homes. Investigators asked anyone who was around five locations , including a car wash, a truck stop and a farm south of town, to report if they saw anything suspicious on July 18.

Last week, Vice President Mike Pence met privately with the Tibbetts family during a visit to Iowa and told them that "you're on the hearts of every American."

At Brooklyn City Hall, city clerk Sheri Sharer said Tuesday was a sad day for the town.

"It never crossed our mind that she wouldn't come home safe," she said.

Willey's group has been publicizing a reward fund that raised around $400,000 for any tip that led to Tibbetts' safe return. He said the fund, which included more than 220 donations from individuals, will now likely be used for any information that helps police catch anyone responsible for her death.

Tibbetts, a psychology major, would have started her junior year this week in Iowa City, about 50 miles (80 kilometers) east of Brooklyn. She was staying with her longtime boyfriend and working while home from school over the summer.