MINNEAPOLIS (WCCO) — “We had no idea how he got on the bus.”

That’s what one chaperone told WCCO after a convicted sex offender was discovered on board with a group of kindergartners.

The 5- and 6-year-olds from Chelsea Heights Elementary School in St. Paul were on a field trip to the Walker Art Museum when a sexual predator hitched a ride to St. Paul by hiding out in the back of the bus.

Maggie Zimmerman says she replays what happened Feb. 28th over and over in her mind.

“As a chaperone, I felt guilty. Like, why didn’t I walk up and down the bus as soon as I got on it? “said Maggie Zimmerman.

After a day of fun, it was time to get on the bus and return to St. Paul.

“Pulled up, we loaded all the kids on, did a count, made sure we had everybody and started down the road,“ Zimmerman said.

Zimmerman says there were two chaperones, a teacher and a teacher’s aide on board.

She says a chaperone separated the 26 students on board from the rows of empty seats in the back of the bus.

“When we got onto 94, our teacher said she saw a man at the back of the bus,“ said Zimmerman.

At first, no one noticed the stranger on board.

“When we looked back again he popped his head up and popped back down,“ Zimmerman said.

The bus driver told the man sit up front, while she called dispatch.

“During the drive back he asked to be let off a couple of times, would she pull over and let him off and she wouldn’t,” Zimmerman said.

When the bus returned to Chelsea Heights, 61-year-old Michael Friedrichs, a registered sex offender, was arrested.

His crimes include engaging in sexual activity with two 15-year-old boys, as well as exposing himself to elementary school children on a Minneapolis school bus in 2011.

School officials told parents how they believe he got on the bus.

“The school told us the footage from the camera shows him prying open the doors at some previous location prior to getting to the Walker when the bus driver was off the bus,” Zimmerman said.

Friedrichs was released from custody and now Minneapolis police have launched an investigation into his registration violation and his conduct on the bus.

Friedrichs was released from prison in April 2018. He is a level 3 sex offender living in downtown Minneapolis homeless shelters.

For now, he is not charged with a crime, but his actions have forced changes in St. Paul’s school bus protocol to include drivers doing a walk through when entering or exiting the bus, making sure seats are empty.