An 18-year-old Arizona student who was charged with sexually assaulting several girls over a five-year span was indicted by a prosecutor on Thursday, the Associated Press reported.

Tyler Kost, of San Tan Valley, has been charged with multiple counts of sexual assault with a minor, sexual abuse with a minor and sexual conduct with a minor. He also faces one count of child molestation.

The victims in the crimes ranged from 12 to 17 years of age in the indictment issued Wednesday, Pinal County Attorney Lando Voyles said. He said the alleged offenses occurred between October 2009 and April 2014.

Kost, born in January 1996, was 13 years old when he allegedly committed the first offense. If convicted of all the charges in the indictment, he would face between 147 and nearly 334 years in prison, the prosecutor said.

Kost's attorney, Christine Whalin, did not immediately respond to requests for comment, according to the AP.

Arrested last week by Pinal County sheriff's deputies, Kost was accused of being a serial sex predator who assaulted at least 18 students. It was not immediately clear why the indictment accused Kost of crimes involving only 11 alleged victims.

Parents were encouraged by Voyles during a press conference to report crimes to authorities.

"Let your children know that we start by believing," he said.

In addition to calling Kost a sexual predator, Sheriff Paul Babeu last week said Kost threatened and harassed victims to such an extent that two fled the state, and one attempted suicide.

"This is a serious and serial sex offender and rapist," Babeu said.

After someone reported that Kost had sexually abused and molested five teenage girls, the Sheriff's Office began an investigation.

Kost lives in San Tan Valley, a bedroom community of about 87,000 named for its location in the foothills of the San Tan Mountains. The area, which lies 45 miles southeast of Phoenix, is known for golf courses and hiking trails, the AP reported.