A Texas mother has been arrested for allegedly buying ammunition and tactical gear for her son who she knew was planning "mass targeted violence" at his school, officials said.

Prosecutors say Ashley Pardo, 33, was aware of her son's obsession with violence, and "intentionally and knowingly aided" his violent goals in exchange for him babysitting his younger siblings.

On Monday, the student showed up to Jeremiah Rhodes Middle School in San Antonio "wearing a mask, camouflage jacket and tactical pants but left shortly after", police said. He was later detained off-campus.

The mother was arrested the same day after the boy's grandmother called police to say Ms Pardo had bought the gun for him.

On Saturday, the boy's grandmother walked into his bedroom to find him playing with live ammunition and a hammer, according to a police affidavit.

She also found a homemade explosive made from a firework, on which the boy had written the name of an Australian man who attacked a mosque in New Zealand in 2019.

The grandmother reportedly told police that Ms Pardo had taken him to a military surplus store to purchase tactical gear, including ammunition magazines, a tactical vest and a helmet.

Police said that the student, who has not been named, told his grandmother on Monday that he was "going to be famous" before leaving for school.

The boy showed up at school "wearing a mask, camouflage jacket and tactical pants", but left shortly after, authorities said. Investigators and school officials did not know where the student was for a period of time, the court document states.

"The student was detained off-campus and charged with terrorism," the school said in a note to parents. The note added that he had been investigated for "some posts the student made online".

Prosecutors say that in January, school officials discovered a map he had drawn of Jeremiah Rhodes Middle School labelled "suicide route". The student then reportedly told officials that he had a "fascination with past mass shooters, including their manifestos".

In April, he was suspended for using a school computer to research previous mass shooting attacks. After time attending an alternative school, he returned to his middle school on 8 May.

Officials said Ms Pardo was aware of the threats made by her son.

"The Defendant expressed to the school her support of (her son's) violent expressions and drawings and does not feel concerned for his behavior," the court document says about Ms Pardo.

She was charged with one count of aiding in commission of terrorism, court records show. She was released from custody on Tuesday after posting a $75,000 (?56,000) bail, and is due to return to court on 17 July.

US police have been increasingly moving to hold parents accountable for their childrens' violence.

Last year, the parents of a Michigan school shooter were convicted of manslaughter for purchasing a gun for their son as his mental health declined.

In Georgia, the father of a 14-year-old is facing charges of murder, manslaughter and child cruelty - the most severe charges against a parent over a US school shooting. (BBC)

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