Updated

Four years after what appeared to be one of the largest single cases of sex assault on US school grounds, it's not clear what happened.

Were at least four middle-school students, ages 11 to 14, victimized by as many as 30 kids in a gym bathroom in Estill, South Carolina?

The sheriff's office investigated and filed an incident report, but a prosecutor never took up the case. Estill Middle School said it couldn't determine what, if anything, happened.

But one grandmother has filed a lawsuit accusing the impoverished district of failing to protect kids.

In a yearlong investigation, The Associated Press uncovered about 17,000 official reports of sex assaults by students in U.S. K-12 schools over four years. AP's analysis was based on state education records and supplemented by federal crime data, where the 2013 incident reported at Estill Middle School stood out.

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If you have a tip, comment or story to share about student-on-student sexual assault at K-12 schools, please email: schoolhousesexassault@ap.org