A St. Paul Public Schools janitor left crude, sexually charged notes and a pair of underwear in a 10-year-old girl's desk at Wellstone Elementary, according to charges.

It's the second time in five years that a district janitor or custodian has been charged with sexual misconduct involving students, and authorities appeared to take the district to task for not making more of an effort to capture the latest offender on surveillance video.

Authorities allege that on April 11, the girl found a note in her desk that said, "I want ur undies Ill [sic] pay u."

She reported it to her teacher, who photographed the note and discarded it.

Oscar Herrera, 23, of Maplewood, was charged Tuesday in Ramsey County District Court with two counts of felony aggravated stalking.

St. Paul Public Schools spokesman Jerry Skelly said that Herrera, who was hired in August 2016, was placed on unpaid administrative leave after his arrest Wednesday.

"We have been in regular communication with the family who was affected by this situation," Skelly said, adding that the district continues to work with police on the investigation.

According to the complaint: On April 17, the girl found a pair of girl's underwear in her desk with a note that said, "try them on." The girl reported it to her teacher again. The teacher took a photo of the note and discarded it.

Police spokesman Steve Linders said the school's principal contacted police on April 17, and that police responded to the school that same day.

The case is the second time since 2014 that a schools staffer has come under fire for alleged sexual misconduct with students. In 2014, then-custodian Walter Happel was accused of peeking at boys in the bathroom, exposing himself and of pressing his genitals onto a boy's buttocks at Linwood Monroe Arts Plus school.

Happel was charged in eight cases and pleaded guilty to abusing his nephew and a neighbor boy. He entered an Alford plea on claims he peeked at a student standing at a school urinal in 2014. The other cases were dismissed.

The charges against Herrera show that school staff obtained a video of him going into the girl's classroom on April 17 with no cleaning supplies. But, authorities said, school officials didn't set up a camera inside the classroom.

"Staff consulted the school district's human resources department about putting a camera in the girl's classroom to find out who had been leaving the notes in the girl's desk," the charges said. "The institutional memory of the Saint Paul Public School District evidently does not extend back to 2014 and Walter Happel as they decided against putting a camera in the classroom."

The girl found another note in her desk on May 23. "$5 for your undies," said the note, which was not destroyed. Authorities believe the notes were written on scraps torn from paper in the girl's desk.

Police met on June 7 with school staffers who had access to the room; five voluntarily provided fingerprints to compare to those recovered on the May note. Herrera, the charges alleged, refused and asked to speak to his supervisor.

Investigators had to rule out students and other staff before executing a search warrant on June 29 to collect Herrera's fingerprints, DNA and writing samples, Linders said.

"There was a lot of investigation … because initially it wasn't clear who was putting the notes in the desk," he said.

A police forensics report completed July 12 connected Herrera to the April 17 note.

Herrera declined to speak with authorities about the case.

Police said there are no other investigations connected to the case, and that although no other victims have been identified, anyone with more information can call police at (651) 266-5685.

Chao Xiong • 612-270-4708

Twitter: @ChaoStrib