On Tuesday (Nov. 1), documents were filed against 17-year-old Iowa students Jeremy Goodale and Willard Miller, who are accused of murdering their 66-year-old Spanish teacher, Nohema Graber. According to prosecutors, the incident happened last year when the instructor gave them a bad grade.

Authorities say the Fairfield High School teacher was beaten to death with a baseball bat because Miller was frustrated about his GPA being negatively impacted. Until this week, it was unclear why the two students carried out the deadly assault against Graber. The Associated Press obtained copies of the recent court filings. In them, Jefferson County Attorney Chauncey Moulding and Assistant Iowa Attorney General Scott Brown said, “The poor grade is believed to be the motive behind the murder of Graber which directly connects Miller.”

He reportedly told investigators that he had nothing to do with her disappearance. However, Miller “later stated he had knowledge of everything but did not participate.” Officials discovered conversations on Miller’s Snapchat that were used to “identify Goodale’s admissions that he acted in concert with another person to bring about Graber’s death.” Miller’s lawyer, Christine Branstad, is currently trying to have that evidence dismissed from the case.

Prosecutors claim the students stalked Graber until they were familiar enough with her routine to catch her on a daily walk. According to NBC News, a witness saw two young males driving the teacher’s van away from the path she frequented while exercising.

On Nov. 2 of last year, the instructor was reported missing. The very next day, her body was found under a tarp, along with a wheelbarrow and railroad ties. Although the students were both 16 years old when they allegedly committed the crime, they will be tried as adults. Goodale’s court proceedings start on Dec. 5. Miller’s trial will begin on March 20.