A US teenager accused of planning a mass shooting at his former elementary school was arrested after sharing a story he wrote describing the attack, police say.

Alex Ye, 18, was arrested in Maryland after an investigation by the FBI and Montgomery County police.

Investigators said Friday that the high school student wrote a 129-page “memoir” in which he described attacking a school in the hopes of becoming famous.

Ye shared the document with another person, who reported it to police.

Ye, a resident of Rockville, is charged with threats of mass violence and faces up to 10 years in prison.

“In the document, [Mr] Ye writes about committing a school shooting, and strategizes how to carry out the act,” police said in charging documents.

The witness who shared the alleged attack plans with police had been sent a chapter of the story on Instagram by Mr Ye. The two previously met at a psychiatric facility.

The witness informed police that there were “striking similarities between the main character” and Mr Ye. The story centres on a character who is transgender and bullied in school, officials said, adding that the witness said elements were pulled from Mr Ye’s life and not fictional.

Officials said the story, which they describe as a “manifesto”, is evidence that he contemplated attacking his former elementary school and high school with a semi-automatic assault rifle.

Investigators also searched his family home and obtained handwritten drawings of violent shooting scenes, and online chat logs in which he threatens mass violence.

In a news conference Friday, Montgomery County Police Chief Marcus Jones said the arrest “underscores the value of community engagement and the ‘see something say something’ approach”.

He added that Mr Ye's threats were taken more seriously than the 140 other threats the school district has received this school year alone.

“This case is entirely different,” he said, adding that the document “takes it to a different level”.

Montgomery County States Attorney John McCarthy praised Maryland’s gun laws – among the strictest in the country – for preventing Mr Ye from obtaining a gun.

He noted that the suspect had posted online about the difficulty of getting guns, and praised investigators for preventing ‘’potential tragedy from unfolding”.

Mr Ye, who was born female as Andrea Ye but now uses male pronouns, previously was sent to hospital for one month in 2022 for threatening to “shoot up school”. According to BBC news partner CBS, he was supposed to be released in January 2023 but was considered a threat to himself and others still. He later was referred to a crisis centre.

As a result of the probe, police increased security at local public schools, although they said he had not attended his high school since the autumn of 2022 and had been studying online. (BBC)

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