SUN PRAIRIE, Wis. – A “suspicious” package dropped off in front of Sun Prairie High School on Monday prompted a lockdown, and the cancellation of classes and after school activities.

Sun Prairie police and fire departments descended on the school Monday after it was discovered that a school-aged person dropped off a box the night before with the school’s address scrawled on top, WKOW reports.

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Authorities told the media the package was deemed “suspicious” because of when it was dropped off, and because school officials did not recognize the person who dropped it off. Officials were also concerned because they could not see inside the re-used postal box, according to the Wisconsin State Journal.

Police evacuated the high school, sending students there for summer classes home for the day, and canceled afternoon activities as the Dane County Sheriff’s bomb squad investigated the package with a robot.

Students who rode the bus were sent home early, but those who drove and parked in the parking lot were kept from their vehicles. Some parents picked up their children behind the school, according to the news site.

“These situations are stressful when you don’t have the right information,” superintendent Brad Saro told WMTV. “We’ve got great security, and we also have investments in all kinds of safety protocols, but when the situation comes up certainly you want your people to be able to meet the situation and keep kids safe.”

Police spent five hours sweeping the school grounds and examining the package. Once the bomb squad gave the all clear, officials retrieved the package and looked inside.

The “suspicious” package turned out to contain two high-end calculators and a textbook.

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“It’s school property,” Sun Prairie Police officer Jon Lothe told WMTV. “We think it was a student who just found them and was probably dropping them off.”

“There was no criminal intent. They didn’t do anything wrong. They didn’t have a note attached to it, they didn’t call and make a threat. They just dropped off school property,” he said. “The doors were probably locked and they dropped them off where somebody could find them.”

The school re-opened later Monday evening for a school board meeting, according to the news site.

The episode in Sun Prairie is the latest in a string of incidents involving an extensive police response to items deemed “suspicious” by school officials.

In March, an Atlanta middle school evacuated and called in the Department of Homeland Security over a “suspicious” backpack. After establishing a perimeter and combing the campus for bombs, police officials examined the backpack and found homework inside, the Atlanta Journal Constitution reports.

And in January Miami Beach’s Biscayne Elementary also evacuated students and called in the bomb squad to investigate a “suspicious” backpack, which police eventually detonated just to be safe. Students huddled in another area of campus and news helicopters hovered overhead as the authorities “rendered it safe,” NBC Miami reports.

The backpack turned out to be a normal backpack, with nothing threatening inside, though police refused to divulge what they destroyed.