Jeremy Goodale and Willard Miller, both 16, have been in jail on a $1 million cash bond since being charged with first-degree murder in the death of 66-year-old Nohema Graber, their high school Spanish teacher.

The documents, filed by Jefferson County Attorney Chauncey Moulding on Thursday, said the teens had surveilled Graber’s patterns, allowing them to attack her while she was on her daily walk.

Graber’s remains were discovered on November 2 at Chautauqua Park in Fairfield, Iowa. According to authorities, she experienced “inflicted trauma to the head,” and was hidden under a tarp, wheelbarrow and railroad ties.

Moulding’s documents assert that Goodale and Miller initially dragged Graber’s body into the woods, then returned to more effectively hide her body. Their motive in the alleged murder is not yet known.

The two have pleaded not guilty, with their lawyers arguing to move their cases to juvenile court. Christine Branstad, Miller’s attorney, argued that Miller had not had any prior involvement in the district court and was only 16 when the crime occurred.

However, Moulding argued that the “nature of the allegations” would require them to be tried as adults, as they are “serious felony” charges. He said he believes there is no way they could be rehabilitated in the short time before they would age out of the juvenile court system and be released.

Hearings for the attorneys’ requests to move to juvenile court are set to begin on January 27.

Moulding argued that trying Goodale as an adult is the only appropriate plan because he would be released at age 18, in less than 24 months, if he is tried and convicted in the juvenile court system.

“This prosecuting attorney cannot fathom any combination of programming at any Iowa juvenile facility which could appropriately treat or rehabilitate the defendant if adjudicated as a juvenile,” he said.

Both teens attended Graber’s Spanish class at Fairfield High School, where she had taught since 2012.

Moulding charged them as adults with first-degree murder and conspiracy to commit murder. The sentence in Iowa would be life in prison for first-degree murder as an adult, although a 2016 Iowa Supreme Court ruling requires juveniles to have a chance of release when given life sentences.

The Associated Press contributed to this report.