St. Paul, Minnesota police arrested and later released a convicted sex offender last week after the man allegedly hid on a school bus hauling kindergartners on a field trip to Minneapolis’ Walker Art Center.

Maggie Zimmerman, a parent chaperone on board the bus with 26 kindergartners from Chelsea Heights Elementary in St. Paul, told MPR News a teacher initially spotted the man crouched down in the back as the group set off back to St. Paul on Feb 28.

“She saw him pop up and then pop back down and we looked and didn’t see anybody and then he kind of peaked over the seat again,” Zimmerman said.

The bus driver summoned the man to a front seat and called police. The man, later identified as 61-year-old convicted sex offender Michael Charles Friedrichs, attempted to persuade the driver to let him off the bus, but the driver refused, Zimmerman said.

St. Paul Police arrested and jailed Linders, then released him after determining he’s registered as a sex offender in Minneapolis. St. Paul Police spokesman Steve Linders said Friedrichs claimed he was confused at the art center when he accidentally boarded the wrong bus.

School officials allege Friedrichs boarded the bus while it was empty, and students were never left unsupervised.

St. Paul Public Schools issued a statement about the incident and reached out to parents of students involved, WCCO reports.

District officials will conduct an investigation “to learn more about what happened and to strengthen our bus safety procedures,” the statement read. “SPPS has also informed its school bus contractors about the incident to reinforce our required safety protocols.”

Parents, meanwhile, are questioning whether that’s enough.

“How many buses has he gotten on and off of without people knowing?” Zimmerman questioned. “How many more buses will he try to get on? In theory if he gets hammed for some probation violation and then serves another short sentence, as soon as he gets out I feel confident he’ll do the same thing.”

Friedrichs’ criminal history seems to support that assessment. He pleaded guilty to criminal sexual conduct in 2004, 2011 and 2016. The 2016 conviction was for molesting two boys at a hotel swimming pool in St. Paul, and he just got out of prison in April for failing to attend sex offender treatment related to that incident, MPR reports.

The 2011 conviction came after Friedrichs snuck onto a Minneapolis school bus and masturbated near students, according to the Minneapolis Star Tribune.

Investigators recovered Friedrichs’ semen from the bus seats in that incident, which also involved elementary age students, MPR reports.

“Everyone is sick to our stomachs and then also livid,” Zimmerman told the Tribune. “It’s kindergartners. This is 5- and 6-year-olds. These kids are babies.”

One of the conditions of Friedrichs’ prison release and 10 years of probation stipulates he’s not allowed to board school buses, and Minneapolis Police have taken over the case from St. Paul to look into charges.

“We have opened up an investigation into this,” Minneapolis Police spokesman John Elder told MPR. “And that includes a possible violation of his registration as well as any conduct that may have occurred on the bus.”