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An Oklahoma school system has reopened after violent threats against a 12-year-old transgender student prompted its closure for two days.

Achille Public Schools, near the Texas border, were shut as local police investigated the threats made in a parents' private Facebook group.

Bryan County Sheriff Johnny Christian said some of the comments came from outside the state.

He said the FBI was contacted to help investigate.

"A lot of the negative comments and what we perceived as threats were outside our county and, sometimes, out-of-state," Sheriff Christian added.

But he said that a man who had confronted the girl's mother in person lived in the small rural community, which is about 160 miles (260km) south of Oklahoma City.

No criminal charges have been filed, but the sheriff said the girl's mother has filed a restraining order against that man.

, one local parent wrote of the student in a Facebook group: "If he wants to be a female, make him a female. A good sharp knife will do the job really quick."

Others referred to the girl as "this thing".

Others called for the student to be attacked, with one suggesting that it was "open hunting season" on transgender people.

The girl's mother told local media her daughter has identified as female for several years.

She said the girl had been falsely accused by a student of peeping under a cubicle while using the girls' toilet.

Sheriff Christian said the girl had used a staff bathroom in primary school. But it was her first week of middle school, he added, and she used the girls' bathroom because she did not know where the employees' one was located.

Superintendent Rick Beene said the decision to suspend classes so soon into the school year was based in part on fears of protests.