CHESTER, Pa. (CBS) — Authorities say a student is in custody in connection with an online threat against three schools in Delaware County.

Overnight, the mayor of Chester announced a 17-year-old girl is in police custody, accused of threatening several schools and thousands of students.

Sources tell Eyewitness News that the student said she was only messing around, but the post had the Chester community on edge as students reported to school Wednesday morning.

The photo, shared on Instagram, threatened shootings at Chester Upland District schools, as well as Chester Community Charter School and Chichester High School.

The post, which includes images of guns, a casket, a knife and horned-faces, closes with the phrase, “All blacks must die.”

It lead to an alert across several campuses and prompted school officials to weigh closing schools today, after two days of weather cancellations.

Mayor Kirkland said all Chester Upland Schools are open today.

Sources say the teen confessed to making the threats at a friend’s sleepover.  Federal investigators tracked her computer IP address and made an arrest.

Shaniyah Vinnie, a student, says “The guns and the faces on there had me scared.”

A parent says, “They should not be posting things like that because of things like this.”

The FBI joined Chester and Chichester authorities and nearby police in the investigation.