By Kristina Dougherty and Joe Holden

PHILADELPHIA (CBS) — An investigation has determined a 13-year-old girl was a participant in the clown social media posts threatening local schools, police say. Authorities say they have the person responsible.

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Police say the girl is a student in Philadelphia schools.

Officials say the teen girl indicated that it was only a prank and she never intended to harm anyone. “At some point in time, these two classmates, decided to take this, post this information on Instagram, threatening a couple of our local school,” said Philadelphia Police Lt. John Stanford.

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Authorities say the “prank” was allegedly committed with a friend at her school. The social media posts named private and public schools as targets of violence.

Over the weekend, Philadelphia Police and the School District announced a joint probe with Homeland Security to track down the source of the threats.

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The Philadelphia Police Department and the School District’s Office of School Safety and Federal Law Enforcement partners say they will continue working in collaboration to investigate additional posts.

The Philadelphia School District and the Archdiocese announced increase security presence at schools.

“It’s a threat absolutely and I’m glad the cops are taking it seriously, but I’m not sure anything will actually come of it,” said Roman Catholic student Christian Dellavella.

“I think people just see the clown stuff and just like, try to imitate it. I don’t think it’s like a big organized group,” Roman senior Matthew Grueninger said.

Police are now consulting with the district attorney’s office to determine if charges are appropriate.

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“This is not a laughing matter,” Stanford said. “This is not a game.”