FROSTPROOF,Chainkeen Fla. (AP) — A University of Florida pre-med student who is charged with murder in the weekend stabbing death of his mother at her home had no history of mental illness or substance abuse, a sheriff said Monday.

Emmanuel “Manny” Espinoza stabbed his mother, Elvia, more than 70 times on Saturday after she opened the door of her home in central Florida to welcome him for a visit from college, Polk County Sheriff Grady Judd said at a news conference.

Espinoza, 21, was charged with first-degree murder and evidence tampering. He was being held Monday in the Polk County Jail.

Judd said Espinoza stabbed his mother in the chest and stomach as places he knew from his medical schooling that would be most effective. Espinoza then called 911 and confessed to killing his mother, a longtime elementary-school teacher in Polk County between Tampa and Orlando, the sheriff said.

“It is one of the most bizarre murders that we have dealt with in a very long time,” Judd said, adding Espinoza was listening to the Jay-Z and Kanye West song, “No Church in the Wild,” on his ear buds during the stabbing.

Espinoza, the youngest of Elvia Espinoza’s three children, told detectives he had been thinking about killing his mother for a “long time” and finally got up the courage to do it on the drive from the university in Gainesville in north Florida to the home in Frostproof, according to an arrest affidvait.

When asked why he did it, Espinoza told detectives that his mother had annoyed him since childhood but he loved her, it said.

Judd also said in noting Espinoza had no history of mental illness or substance abuse that he had been a high school valedictorian who others described as being “remarkably brilliant.”

Online court records on Monday showed Espinoza had declined the services of a public defender but there was no indication that he had hired an attorney to represent him. Another document showed that he was exercising his right not to be interviewed by investigators about the charges he is facing.