Officials said Tuesday afternoon that a person of interest is in custody, but the gun remains missing.
School will be back open Wednesday.
Arlington Heights police said a school resource officer working at Forest View Educational Center took the gun off while using the restroom Monday and realized it was gone shortly after students had already left for the day.
Officers quickly checked surveillance and saw only a small number of people went in and out of that restroom. But even after a large search, including K-9 units trained to detect weapons, no gun was found.
Even though they confirmed it's not inside the building, out of caution, classes were canceled Tuesday.
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Police are calling the situation serious and embarrassing as they work to find both the weapon and whoever may have taken it.
The officer involved has been placed on leave.
Dartagnan Young lives across the street from the school.
"Super concerned because that's a firearm that's out there. My working theory is maybe one of these kids over here has it," Young said. "I hope they find it and no one gets hurt, you know."
Desmond Davis, a parent who has a student at the elementary school down the street, called the situation troubling.
"The whole community is in danger," Davis said. "We don't know the mindset or the capability of this person with a gun."
Police are asking anyone with information to come forward. A few students from other schools went inside the building for Advanced Placement, or AP, testing; everyone else in Tuesday is staff or police.