James Cornelius IV, 22, of Woodstock.

A 22-year-old child sex offender was arrested for allegedly entering Woodstock High School during the school day on Monday, police said.

Faculty members told the Woodstock Police Department’s School Resource Officer that is assigned to the Woodstock High School, 501 West South Street, that they believed a registered sex offender had entered the school building at the end of the school day on Monday while students were still there.

Police reviewed the school’s surveillance videos and identified the man as James Cornelius IV, 22, of the 300 block of West Calhoun Street in Woodstock, who is a registered child sex offender. Police said Cornelius entered the school building from 4 p.m. to 4:10 p.m.

The McHenry County State’s Attorney’s Office obtained an arrest warrant for Cornelius for one count of presence within a school zone by a child sex offender.

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On Wednesday, Cornelius was arrested in the 1200 block of Walden Oaks Drive by Woodstock police officers.

Cornelius remains held in the McHenry County Jail.

“The Woodstock Police Department worked collectively with the faculty and administrators at the Woodstock High School and School District 200 during the entirety of this investigation,” Woodstock police said.

“The Woodstock Police Department continues to partner with School District 200 in making safety a top priority for all students and faculty.”