SAN JOSE, Calif. (AP) — Police in Northern California say a 12-year-old girl is believed responsible for sending a threatening tweet against schools in San Jose's Alum Rock Union School District.

The Mercury News reports Friday police officers served a search warrant at the girl's house in San Jose Thursday night.

The San Jose Police Department says the investigation is ongoing.

The threatening tweet posted Wednesday says, in part, "You guys look so innocent and say you have a no bully zone but you literally make me want to shoot up your (expletive) schools."

Superintendent Hilaria Bauer acknowledged the troubling tweet in a letter to parents and guardians Wednesday.

A surge in threats against schools has prompted numerous arrests in the wake of the Florida school shooting.

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Information from: San Jose (Calif.) Mercury News, http://www.mercurynews.com