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More than a dozen students were sent to the hospital after a mysterious odor swept through part of Blevins Middle School in Fort Collins.

The incident was first reported at about 12:30 p.m., initially from a laboratory classroom. Poudre Fire Authority investigated for nearly six hours and didn't locate the source. Investigators plan to return to the school on Saturday.

"When the firefighters arrived on scene they found the 12 students in front of the school and the remaining students in the process of being evacuated," said Poudre Fire spokesman Patrick Love.

Students say they were told it was fire drill and to evacuate.

"We were out there for 15 minutes. We were thinking something was wrong," says 8th grader Sean Papile.

The army of firefighters, police and paramedics made them question--and worry.

"When we came out we had no idea what was going on, so it was a little freaky for us," says 6th grader Jack Sexton.

Teachers and students say the odor came from a science lab, and whatever it was it made several of them sick.

"We had about 15 students taken to the hospital with symptoms of illness. We don't believe it's anything serious. It's as a safety precaution," says Poudre School District spokeswoman Danielle Clark.

"I started sneezing and my eyes started watering...then we went to the nurse and half the class came in saying they were feeling pretty much the same symptoms," says 6th grader Fiona Papile.

She was one of the students who went to the hospital. Other students experienced headaches, nausea and vomiting, according to Poudre Fire spokesman Patrick Love.

Fiona says she didn't smell anything but says other kids did.

"People were saying they smelled something but nobody can agree on what it smelled like," she says.

So firefighters had to try to figure it out by going inside the school with protective gear and air tanks to breathe.

There was no way the kids were going back.

"After the evacuation they did not let anyone back in. Those who rode their bikes in had to ride without helmets, or homework or backpacks," says Sean.

"I left my jacket and my trumpet and my kindle in my locker so I'm really sad," says Fiona.

But some kids are okay with no homework for the weekend.

"Happy. 'Why?' because I don't have to do it," says Sexton.

And now they have a good excuse.

All students were treated at Poudre Valley Hospital and released. One adult at the school also drove him or herself to the hospital.

Poudre fire says it suspects the odor came from an airborne chemical that dissipated. Their air testers did not pick up on anything.

The school will be running its air conditioner through the night to further clear the air.