Brooke Bestell, 20, told the New York Times that she turned Rivera, 24, down for a date.
“He would just stare. He wouldn’t really, like, talk,” she said. “Something about him was off.”
Bestell added that he would send her repeated Facebook messages in the middle of the night.
“Just over and over, like every week or so, he would message me again,” she said. Her most recent messages from him were from mid-June.
Bestell said that two of her friends also received messages from him online.
“I don’t know how many other girls he probably was trying to talk to,” she said.
Rivera approached Tibbetts as she was running on July 18, and she threatened to call the cops, according to an arrest affidavit obtained by Oxygen.com.
The exchange enraged Rivera, according to the documents, and he claims he blacked out before coming to at an intersection with Tibbetts' body in the trunk of his car.
Rivera said he does remember seeing Tibbetts’ headphones in his lap or removing her bloodied body from his trunk.
“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 says.
The findings of a preliminary autopsy revealed she died from “homicide resulting from multiple sharp force injuries, according to the Iowa Department of Public Safety.
Police hinted that Rivera may have known Tibbetts.
"He tells us that he had seen her before but beyond that, I can't tell you anymore," Rick Rahn of the Iowa Division of Criminal Investigation said, according to CBS News.
Rivera is being held on a $5 million cash bond. His arrest led to a national uproar and controversy surrounding his immigration status.
[Photo: Iowa Department of Public Safety]