An 11-year-old girl’s diary entry led to the arrest of a 75-year-old substitute teacher for Northside Independent School District, according to an arrest affidavit.

Luis Lopez is accused of sexually assaulting the student several times during off-campus tutoring sessions over the summer.

The John Glenn Elementary School student’s diary was discovered by her little sister over the weekend, according to the affidavit. Her little sister then gave it to their great-grandfather, who is also their legal guardian, to read.

The man became ill at what he discovered.

The diary entry included ‘graphic sex writings’ detailing the 11-year-old’s encounters with the 75-year-old tutor at his home.

The affidavit stated the child’s guardian allowed her to go on the off-campus tutoring sessions with the NISD substitute who also waived his $70-per-hour tutoring fee for the family.

She told authorities Lopez took them to his southwest-side home one day over the summer and allowed her to shower in his bedroom. After she took a shower he proceeded to sexually assault her on his bed while her brother was in another room.

The affidavit also shows that Lopez frequently took the child and her brother swimming, to the movies, and out for lunch.

When the girl was interviewed by the Child Safe Advocacy Center she confirmed that Lopez purchased her and her brother clothes, shoes, a cell phone, and even a laptop.

Lopez was arrested on Wednesday at a motel and booked on charges of aggravated sexual assault of a child.

SAPD said it appeared he was getting ready to leave town.

Investigators are now searching for other possible victims.