Police in Arizona say they thwarted a girl's plan for a Newtown-style massacre with the help of Toronto police.

The 16-year-old Red Mountain High School student posted a threat to "gun down her school mates and then kill herself" on a YouTube channel owned by a transgender community in Toronto, according to a news release from Maricopa County Sheriff Joe Arpaio.

"I now literally have a plan of seriously hurting ... killing ... murdering people in my high school," she said in the post, adding that she'd plotted her attack well before last Friday's mass shooting at Sandy Hook Elementary School. "I work at the high school as a student teacher basically ... which is why no one can find out about me somewhat accidentally planning to blow up the school."

The owner of the channel alerted Toronto police, who tracked down the girl, then notified the sheriff's office Wednesday.

The girl admitted to posting the threat when Arizona detectives questioned her at her home, with her parents present, the news release said. She also told them she was afraid she would actually carry out the threat and "f------ kill everyone."

Her parents told investigators the teen has a history of mental health issues, and they'd become alarmed when she recently started asking where the guns were kept in the house. Her father placed trigger locks on the weapons -- a semi-automatic .40 caliber, a .357 revolver and a .22 revolver - and locked them in a gun safe, the sheriff said.

The girl faces one count of computer tampering and one count of threatening and intimidating.