Authorities in Arizona, with the help of Toronto police, have arrested a 16-year-old high school girl who they say was plotting to gun down her classmates and then kill herself.

The girl, who lives in Mesa, Ariz., about 32 kilometres east of Phoenix, is accused of posting a threat on a YouTube channel, saying, "I now literally have a plan of seriously hurting … killing … murdering people in my high school."

The YouTube channel is operated by Ryerson University student Alex Haney, a member of the transgender community in Toronto who notified Toronto police after the disturbing online comment was posted.

Haney, who uses the YouTube channel to post diaries chronicling his experiences as a transgender youth, believed at first the comment was written by an angry young man.

"He was thinking of plans in his head for how to carry out hurting people in his school, and that it would have been easy to bomb the place, but he just didn't have the stuff for a bomb," Haney said, describing the content of the online posting flagged to authorities.

"It could have been a hoax, it could have just been meaningless," Haney thought.

But the university decided not to take any chances, and called Toronto police.

They helped track the online threat to Arizona, and contacted the Maricopa County Sheriff's Office. Within three hours of Haney's initial report, detectives visited the 16-year-old girl's family home in Arizona, where she admitted in front of her parents to posting the threat.

Police seized two computers and also found three handguns in the home. The weapons were released to the girl's father for safe-keeping.

According to Joe Arpaio, the sheriff of Maricopa County, one of the weapons was a semi-automatic handgun.

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

Post alludes to Newtown massacre

In the YouTube post, the girl alludes to the Sandy Hook Elementary School massacre in Newtown, Conn., saying that she had planned her attack well before last week's deadly shooting.

The girl, who also serves as a student teacher at Mesa's Red Mountain High School, wrote in the post, "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. (and yes…it would be super easy…)."

The girl's parents told police that she has a history of mental health issues and had been seen by a psychiatrist. She had recently been asking where the guns were being kept in the house, they said.

Police said the girl admitted she was afraid she would really carry out the plans detailed in the post. She told them she had thought about killing for a while and planned to "cut the wires to the security carts" on campus to slow down response time during an explosion, and to shoot people from the other side of a fence.