By Sandra Rose  | 

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Cook County Jail

A Lollapalooza security guard was arrested for making a false mass shooting threat so she could leave work early.

Janya Williams, 18, was scheduled to work during the four-day Lollapalooza music festival at Grant Park in Chicago last weekend.

According to the Cook County Sheriff's Office, Williams sent her supervisor a TextNow message on Friday afternoon that said, "Mass shooting at 4pm location Lollapalooza. We have 150 targets."

The supervisor contacted the Chicago police, who notified the FBI.

Williams then told the supervisor that her sister saw a similar message on Facebook. The supervisor asked Williams to send a screenshot of the Facebook threat.

Williams allegedly created a fake Facebook account under the name "Ben Scott" then posted a message that read, "Massive shooting at Lollapalooza Grant Park 6:00 p.m." She took a screenshot of the fake post and sent it to her supervisor, who forwarded it to the FBI, prosecutors said.

The FBI traced the original TextNow message back to Williams' IP address and iCloud, prosecutors said.

Under interrogation, Williams admitted making the false threats "because she wanted to leave work early," prosecutors said.

Williams was arrested and charged with making a false terrorism threat, a felony. She is being held in the Cook County Jail on $50,000 bail.