A registered sex offender from Glen Ellyn was sentenced Monday to 36 years in state prison for sexually abusing a mentally disabled girl several times over the course of about three years.

Gerard Schlaiss, 51, pleaded guilty and was sentenced by DuPage County Circuit Court Judge Robert Anderson under an agreement with prosecutors, assistant state’s attorney Liam Brennan said.

Glen Ellyn police arrested Schlaiss in September after he was seen kissing and inappropriately touching a 16-year-old girl outside a liquor store on Roosevelt Road. The girl lives with her parents in the same apartment complex where Schlaiss lived.

During the course of their investigation, authorities learned that Schlaiss had molested the girl about half a dozen times beginning in 2004 when she was 13 years old, said Brennan, supervisor of the state’s attorney’s child abuse unit.

Schlaiss was required to register as a sex offender after he was convicted in 1989 of aggravated criminal sexual assault in Lake County. The victim in that case was a 13-year-old girl.

He was sentenced to six years in prison for that crime and served about half of that sentence, Brennan said.