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FLORENCE, Ariz. (AP) — A prosecutor announced an indictment Thursday against an 18-year-old Arizona student charged with sexually assaulting several girls over a nearly five-year span.

Charges against Tyler Kost of San Tan Valley include multiple counts of sexual assault with a minor, sexual abuse with a minor and sexual conduct with a minor. Kost also faces one count of child molestation.

Pinal County Attorney Lando Voyles said the victims in the crimes alleged in the indictment issued Wednesday ranged from 12 to 17 years of age.

Voyles said the alleged offenses occurred between October 2009 and April 2014.

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

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

Pinal County sheriff's deputies arrested Kost last week, accusing him 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.

During a news conference, Voyles encouraged parents of sexual assault victims to report the 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.

The investigation was initiated after the Sheriff's Office received a call March 25 from someone reporting that Kost had sexually abused and molested five teenage girls.

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.