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An Arizona high school student was arrested after posting a Snapchat image of a crowded school hallway with the message "planning the school shooting," police said.

The 16-year-old student, who was not identified by police because he is a minor, posted two photos to his Snapchat account of a crowded school hallway on Aug. 31, one with the text “Planning the school shooting” along with several smiley faces and another with the caption “In the office again,” the Phoenix Police Department said in a statement.

Kaylee, a high school student in Colorado, saw the first message on the popular social media app, but said she did not know who the student was.

A screenshot of the Snapchat post. Kaylee / via KUSA

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"It freaked me out," she told NBC affiliate KUSA.

Kaylee, 16, took a screenshot of the photo and sent it to her mother, who told her to alert authorities, according to KUSA.

She informed the school resource officer at her Northglenn, Colorado, school about the disturbing post.

Saving a Snapchat image notifies the person who sent the message, and the teen who authorities say posted the threat soon reached out to Kaylee.

After a brief text message exchange, the male student told Kaylee he attended Sierra Linda High School, more than 800 miles away in Phoenix, Arizona, according to KUSA.

Phoenix police arrested the teen, who admitted involvement in the incident but told officers it was a prank, according to the statement.

The student was detained for one count of creating a hoax, a class 4 felony, according to the statement.

Kaylee said she informed authorities in case the student’s threat against his school was credible.

"I didn't want to assume it was a joke because if I would have woken up today and seen that he really did shoot up a school, I know how bad I would have felt if I know I could possibly have stopped it," she said.