A 67-year-old man who is a registered sex offender was charged Wednesday with exposing himself in front of three teenage girls at an Orange County library, authorities said.

Robert Bruce Engel pleaded not guilty Wednesday to a charge of indecent exposure, according to Orange County Superior Court records.

Engel is charged with entering the Rancho Santa Margarita Library on May 9 and exposing himself as he sat across from the three girls, the Orange County district attorney's office said.

The girls reported the incident to a librarian, but the man had left by the time police arrived. The librarian recognized Engel when he returned the following week and called authorities, the district attorney's office said.

Engel was being held in lieu of $20,000 bail. He is scheduled for a pretrial hearing May 26.

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-- Robert J. Lopez
Twitter: @LAJourno

Photo: Robert Bruce Engel. Credit: Orange County district attorney's office