As a flurry of first responders swept in, panicked students, staff and parents tried to figure out what was going on.
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"I thought it was a drill, and then I got nervous and I was like 'oh my God, what's going on? Like, what do I do?'" said Lahnee Lopez-Ciura, a student.
The Maine West sophomore recounted the terrifying moments after an active shooter alarm was set off at the school just before 10 a.m.
"You see the helicopter fly over, and the sounds of the sirens and your friends are crying. That was like, this is the high school experience you don't want to experience," Lopez-Ciura said.
North suburban school on lockdown after false 'active shooter' alarm
Des Plaines police told ABC7 that officers immediately entered the school when they arrived on the scene and never detected an active threat or armed person.
Maine Township School District leaders said they believe the active shooter alarm was either accidentally triggered or set off by a short in the system, but the exact cause is still unknown. A district spokesman said the school followed protocol. Some classes evacuated and others sheltered in place.
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"Most likely barricaded doors, got shut off the lights, closed the blinds, probably got into the safest spot they could get into within the building and then waited," said Maine Township School District Spokesman Brett Clark.
"Right away I'm like, 'okay, oh my gosh, are you okay?" said Dede Lopez-Avina, a parent.
Lopez-Avina picked up her daughter, Lahnee, at a reunification site at a community center just over a mile away from Maine West.
She said she's glad this nightmare is over.
"I do feel comfortable and confident that they did their best in getting the kids and staff out and making sure everyone was safe," Lopez-Avina said.
The Maine Township School District is offering counseling services for students and staff shaken by the false alarm.