An “illegal alien” is in custody for the murder of Iowa college student Mollie Tibbetts, according to police officials.

Cristhian Rivera, 24, was charged Tuesday in Poweshiek County after being brought in without incident by local cops.

Investigators spoke to him at length about what happened on July 18 — when he allegedly abducted Tibbetts — and revealed what he said during an afternoon press conference.

According to officials, Rivera told cops that he saw the young woman running, circled around a few times in his car, and eventually approached her.

He got out an ran up behind Tibbetts and accidentally spooked her — causing the 20-year-old to panic, officials said.

Rivera told cops that he then blacked out, and didn’t come to again until after Tibbetts was already dead — at the intersection in rural Poweshiek County where investigators found her body.

Cops would not say when the young college student was left there.

Rick Rahn, special agent in charge at the Iowa Division of Criminal Investigation, told reporters that federal officials were able to confirm Rivera’s status as an undocumented immigrant after he was brought into custody on Monday.

“He’s an illegal alien, been in this area for four to seven years,” Rahn said.

Investigators were able to link Rivera to the missing person case after catching his black Chevy Malibu on surveillance camera the day Tibbetts disappeared. He was allegedly tailing her as she ran.

“He followed her around,” Rahn said. “Circled around a couple times, located her, then began to interface with her.”

Investigators said they got the surveillance video last week and immediately began working to find Rivera.

“We were able to track [his car] and the routes in which he took,” Rahn explained. “He was one of the last ones to see Mollie running.”

After locating him, cops said they conducted lengthy interviews with Rivera and eventually got him talking.

“It seemed that he followed her and seemed to be drawn to her on that particular day,” Rahn said, “and for whatever reason, he chose to abduct her.”

Rivera is currently employed, though officials refused to say what he does for a living. He has been charged with first-degree murder.

It’s still unclear how Tibbetts was killed, but authorities expect to find out in the coming weeks as the autopsy wraps up. It’s scheduled to begin on Wednesday.