LAKE ZURICH, Ill. (CBS) — Lake Zurich High School was first placed on soft lockdown Wednesday afternoon due to the discovery of a shell casing, but a hard lockdown followed after the school reported a student threatened violence.

The Lake Zurich Community School District 95 first reported that just before 12:30 p.m. that a soft lockdown had gone into effect. At the time, the district reported there was no threat reported, and no gun had been found – just a shell casing. Still, in an abundance of caution, students and staff were placed on a soft lockdown.

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Classes continued, but students were required to remain in their assigned locations and no visitors were allowed at the school. After an investigation, school officials and Lake Zurich police found there was no threat, and the lockdown was lifted, the district said a little over an hour later.

But afterward, the school reported there was actually an imminent threat posted by a student that claimed an act of violence was imminent at the school. The high school immediately went into hard lockdown and 911 was called.

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Nearby May Whitney Elementary School was also placed on hard lockdown for a short time, the district said.

A student was arrested by the Lake Zurich Police Department, and police also swept the inside and outside of the building.

The hard lockdown has been lifted, but a soft lockdown will continue for the rest of the day. Student s will be dismissed from hallways at 3:21 p.m. as usual.

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“Please know that many students and staff are understandably upset by today’s incidents,” the district said. “We will be developing a plan to provide support.”

CBS 2 Chicago Staff