Pinal County Attorney Lando Voyles announced the indictment against Kost, 18. Voyles said that Kost faces adult charges even though some of his alleged crimes occurred before his 18th birthday.
"Once an individual turns 18 they now face the consequences of the adult system," Voyles said. "That includes anything that may have happened prior to that."
Kost, who was suspended from Poston Butte High School in San Tan Valley after his arrest last week, allegedly committed his first assaults in October 2009, and continued until shortly before his arrest. Voyles said the indictment handed up Wednesday by a grand jury involves 11 girls ranging from age 12 to 17.
Amy Fuller, superintendent of the Florence Unified School District, said Kost was allowed to take online classes in February, after he and his parents said girls were bullying him. Fuller said she did not believe he had been bullied.
Joey Hamby, Kost's defense lawyer, has accused Pinal County Sheriff Paul Babeu of picturing Kost as a serial rapist without any evidence. Babeu said last week that text messages and other online exchanges between Kost and his alleged victims will show that he threatened at least one girl.
A parent of one of the alleged victims has also said that a staffer at a Planned Parenthood deliberately failed to code the cause of the girl's pregnancy as a sexual assault. The clinic is legally required to report all rape allegations.
"The counselor intentionally miscoded the assault as a consensual encounter," said Pinal Sheriff's officials, quoting the parent in documents submitted in an effort to deny Kost bail. "The counselor told them they did not want the hassle of having to report the assault to law enforcement, as they were a mandatory reporter."
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