A former Minneapolis charter school teacher who is already in prison for sexually assaulting four boys he coached has been indicted on charges of sexual assault of 12 other children all under the age of 13.

The Hennepin County Attorney's Office announced Thursday that a grand jury indicted Aaron Hjermstad of Brooklyn Center on 12 counts of criminal sexual conduct. Hjermstad taught at several charter schools in the Twin Cities — including the Mastery School and Harvest Best Academy. He also coached boys basketball. Many of his victims were either children he had coached or relatives of those children.

The 12 counts each carry a possible sentence of life in prison, with the possibility of parole.

Hjermstad, 45, was sentenced to 12 years in prison on four charges of criminal sexual conduct in 2022. He is being housed at the state prison in Moose Lake and his anticipated release date had been Nov. 21, 2029.

The new charges detail abuses that occurred between 2013-2021. When Hjermstad was originally charged with four counts of criminal sexual conduct in 2020, he fled Minnesota. He was arrested by police in Idaho who found "thousands of videos of him assaulting children, some at his Brooklyn Center home." That data was turned over to the Minnesota Bureau of Criminal Apprehension, which began an investigation into Hjermstad. The victims in those videos include the 12 children in the new indictment.

The hiring of Hjermstad at the Mastery School, which merged with Harvest Best in 2021, is at the center of a still-pending Minnesota Supreme Court decision. One of Hjermstad's minor victims, through his mother, sued Hjermstad, the Mastery School, Harvest Best Academy and Hospitality House in 2020. She claimed the Mastery School was negligent for failing to follow its own hiring protocols.

Hjermstad had worked at Excell Academy — a Brooklyn Park charter school — but he was put on administrative leave after sexual assault allegations were made against him by a student and the school didn't renew his contract. Not long after, the Mastery School's principal hired Hjermstad. He cleared a background check, but the school didn't contact Excell or other previous employers, according to court records.

The state Supreme Court was asked in 2023 to determine whether districts and charter schools are liable when someone with a problematic past is hired and goes on to abuse students. The court has yet to issue an opinion.

Hjermstad was a gym and health teacher at the Mastery School and a basketball coach at Hospitality House Youth Development and Harvest Best Academy.

"The number of children who were sexually assaulted by Hjermstad is horrific," Hennepin County Attorney Mary Moriarty said in a prepared statement announcing the indictment. "We will seek lengthy prison sentences to keep children in our community safe."

Harvest Best Academy said in a statement that "The allegations in the indictment are deeply disturbing, and Harvest Best Academy is heartbroken by the harm Aaron Hjermstad has caused to our students and other victims."

This is the second case this month where a former teacher at Harvest Best has been accused of sexually assaulting minors.

Last week, Abdul Jameel Wright, 38, was charged with first-degree criminal sexual conduct after he allegedly groomed and sexually assaulted a 14-year-old student from his English class starting in 2016. That same year, Wright had been named the Minnesota Teacher of the Year.

Briana Bierschbach and Mara Klecker contributed to this story.