Hawaii Teen Was Shackled in School Bus, Forced to Smoke Meth Before Good Samaritans Helped Her Escape: Police

A man in Hawaii allegedly abducted and sexually assaulted a 15-year-old girl, kept her shackled in a school bus and forced her to smoke methamphetamine.

Duncan Mahi, 52, has been charged with two counts each of kidnapping, terroristic threatening in the first degree, robbery in the first degree and one count each of methamphetamine trafficking and sex assault in the first degree.

"This incident is a very shocking thing to happen," prosecuting attorney Kelden Waltjen tells PEOPLE. "It's often taught that this is the unicorn type of situation. The suspect is very often either closely related or known by the victim and this is not the case that we have here. This is a very unusual set of circumstances."

Mahi was indicted by a grand jury Wednesday.

According to a probable cause affidavit obtained by PEOPLE, Mahi approached the teen and her 15-year-old boyfriend at about 3 p.m. on Sept. 16 while the couple was hanging out at Anaeho`omalu Bay in Waikoloa, Hawaii. Mahi allegedly pulled out a knife and demanded money.

Mahi allegedly took their money and cell phones before he forced the girl to zip-tie and tape her boyfriend's hands and legs. He then forced her to stuff his shirt in his mouth, place a towel over his head, and secure both with tape, meanwhile threatening to kill her if he escaped his bindings.

Mahi then allegedly led the teen through Kiawe trees and the lava fields to a white Honda SUV parked in a nearby beach parking lot. He allegedly made her put on a hat and disposable mask before driving off. While driving, he allegedly forced her to smoke methamphetamine twice.

According to the affidavit, at his home, Mahi forced the teen inside a yellow bus that was located behind the main house. There, he handcuffed her left ankle to a cable that was attached to the bus, according to the affidavit.

Escape at Restaurant: 'Brave and Smart Girl'

Investigators said the following morning, the teen was able to convince Mahi to take her to a local restaurant in Hilo Bay. And that's where she was able to make her escape.

"She's a very brave and smart girl," Waltjen tells PEOPLE. "The fact that she was able to critically think about what she had to do in order to survive — I think she went into survival mode — and I really just have to commend her for her bravery and her determination to live."

A witness later told police he heard "screaming and yelling" coming from the alleyway nearby.

The witness told investigators he saw Mahi and the girl "wrestling with each other," and that the "male was on top of the female as the female went between the male's legs, running in the opposite direction."

While the witness tried to stop Mahi from chasing her, Mahi allegedly said, "That's my daughter. What the f--- are you doing? Get out of my way!"

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A man nearby recognized the teen, who had been the subject of an Maile AMBER Alert after her abduction, stating, "that's the girl in the news," according to the affidavit.

"[The witness] was aware of the Maile AMBER Alert at the time so he said to the male that if that is your daughter I'll go find out," the affidavit states. "Upon saying that, the male immediately ran to a car."

Investigators said the witness followed Mahi and took photographs of his car and license plate number.

"I'm really glad that he was, one, a person who's paying attention to what's going on in the community: aware of his surrounding, very observant and was willing to step in and put himself in danger in order to save a complete stranger," Waltjen says. "A lot of people are calling him a hero. Rightfully so."

"It's kind of a theme that's been really catching on as far as in law enforcement nationwide: If you see something, say something. And that's what he did," adds Waltjen. "He saw her, he recognized her and he called out her name, and let people know that's who it was."

Mahi was later picked out in a photographic lineup.

At his home, investigators found a bucket of zip-ties, tape and the handcuffs, the affidavit alleges.

Mahi was taken into custody later that day.

Bail has been set at $2 million.

He is scheduled to appear in court Friday. He has yet to enter a plea. It is unclear if he has retained an attorney.