An underage high school student was sexually assaulted in a bathroom by a knife-wielding 19-year-old who had trespassed onto campus in a suburb of Dallas, according to police.

The incident unfolded on Tuesday afternoon on the campus of Mesquite ISD's Horn High School in the city of Mesquite, Texas.

'The mother of the victim had warned police that Elijah had threatened her child before the attack and said that he would show up on the campus.'

Mesquite police said that the man was able to enter the school despite security measures because someone inside opened a locked door for him. Once inside, he allegedly sought out a female student whom he had previously had a relationship with and forced her into a bathroom.

Police said in a press release that the man assaulted the victim and "used a weapon to force her into a bathroom, where the sexual assault occurred."

The suspect ran off from the school after a teacher realized he wasn't a student and was later arrested at his home in Seagoville, police said. They identified him as 19-year-old Synnaiz Elijah.

A spokesperson for Mesquite ISD explained the security measures taken at the school.

“Exterior doors are locked for a reason. There are only two authorized entrances for visitors to enter Horn High School,” read the statement. “All exterior doors are checked four times every day by campus staff to ensure they are locked and closing properly. There are three full-time School Resource Officers and three security guards on campus every day.”

Elijah is charged with aggravated assault of a child.

An arrest affidavit said that the mother of the victim had warned police that Elijah had threatened her child before the attack and said that he would show up on the campus. She even went to the school to notify campus police just hours before the assault allegedly occurred.

The school said in a letter to parents that the student who allegedly allowed Elijah in to the school would be punished, but some parents were angry that they weren't notified about the incident more quickly.

Mesquite has a population of about 150,000 residents.

A news video about the arrest can be viewed on the YouTube channel for KDFW-TV.

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