A registered sex offender was arrested after entering an empty school bus and then hiding in it while St. Paul kindergarten students boarded it after a field trip last week.

The bus was heading back to Chelsea Heights Elementary School in St. Paul after a Thursday trip to the Walker Art Center in Minneapolis.

The 61-year-old suspect was discovered sitting in the back of the bus during the return drive, prompting the driver to call police, authorities said Monday.

St. Paul police arrested the man after discovering he was a registered sex offender. He told officers he had been at the Walker and boarded the wrong bus, police said.

Minneapolis police have picked up the investigation. Spokesman John Elder confirmed it “is investigating the possibility of a registration violation, which will include any conduct on the bus.”

The Star Tribune generally does not name suspects before they are charged. Court records show the suspect pleaded guilty to fifth-degree criminal sexual conduct in 2011 after accusations that he sneaked onto a Minneapolis school bus and masturbated near students.

“Everyone is sick to their stomachs and then also livid,” said Maggie Zimmerman, a parent who was on the bus Thursday as a chaperone. “It’s kindergartners. This is 5- and 6-year-olds. These kids are babies.”

Zimmerman said the bus was on Interstate 94 when a teacher saw the man pop his head over a back seat. The bus driver, she said, then directed the man to sit at the front until they got back to the school.

St. Paul Public Schools is reassuring parents that the district is also investigating what happened and working to “strengthen our bus safety procedures,” district spokesman Kevin Burns said.

Chelsea Heights Elementary Principal Jill Gebeke told families in a letter that video shows the man boarding the bus after the driver had gotten off, before picking up the students at the Art Center. “At no time were students unsupervised by teachers and chaperones,” she wrote.

As for safety protocols, Gebeke continued, “When a driver enters or exits a bus, they must walk through to verify that the bus is empty and ready for its next route.”