ST. PAUL, Minn. — A registered sex offender on Thursday allegedly hid inside an empty school bus before kindergartners got back on board following a field trip, and was discovered soon after and arrested.

The bus was on the way back to Chelsea Heights Elementary School after a trip to the Walker Art Center, the Star Tribune reports. The suspect, identified by KSTP as Michael Charles Friedrichs, 61, was found to be sitting in the back of the bus. Maggie Zimmerman, a parent who was riding the bus as a chaperone, told the Star Tribune that a teacher saw the man pop his head up above one of the seats. She added that the bus driver told the man to sit at the front of the bus until they arrived at the school. Authorities said that the driver also called the police, according to the news source.

St. Paul Police Department officers arrested Friedrichs after finding out that he was a registered sex offender, Star Tribune reports.(As previously reported, Friedrichs had been arrested in 2011 for boarding a school bus without permission and allegedly performing a sex act. In 2004, he had been convicted of criminal sexual conduct.)

Friedrichs told officers that he had been at the art center and had gotten on the wrong bus. Jill Gebeke, principal of Chelsea Heights Elementary School, told students’ families in a letter that video shows Friedrichs boarding the bus after the driver exited, before picking up the students at the art center. The news source also reports that students were under the supervision of teachers and chaperones at all times. Gebeke also told parents in the letter that when a driver boards or exits a bus, they are required to walk through it to ensure it “is empty and ready for its next route.”

Kevin Burns, a spokesperson for St. Paul Public Schools, released a statement on the incident, which was obtained by KSTP. It said that the district is conducting its own investigation into the incident “to learn more about what happened and to strengthen our bus safety procedures. SPPS has also informed its school bus contractors about the incident to reinforce our required safety protocols.”