Officials at an elementary school in southwest Calgary are left scratching their heads after a mysterious illness affected over a dozen students, sending them to hospital.

Fire officials confirmed they were called to Woodlands Elementary School, at 88 Woodgreen Drive S.W., at about 9:25 a.m. for reports of carbon monoxide concerns.

However, officials have ruled out the deadly carbon monoxide gas in the incident, and they are still working out what it could be that sent 15 people, many of those children, to hospital.

Stuart Brideaux, with Calgary EMS, says that all the patients taken to hospital are in non-life-threatening condition.

He says that all the patients, students between the ages of eight to 10 years old, were complaining of a general feeling of nausea and other non-specific concerns.

"No identifiable causes or sources of illness were found through testing employed at the scene including air and water quality measuring."

Carol Henke, with the Calgary Fire Department, says they've already ruled out a carbon monoxide leak at the school through the use of specialized detectors.

"Early reports were that someone had heard a carbon monoxide alarm. That is actually not accurate. We have used several of our monitors and we've had the Haz Mat team in. They could not find any readings of carbon monoxide. At this point, it's a bit of a mystery."

She says that the students were experiencing some nausea and a feeling of having a queasy stomach, a common symptom of carbon monoxide poisoning.

The students were all assessed and transported to hospital as a precaution, and the school was also evacuated as part of the precaution.

One parent tells CTV Calgary that their child apparently became ill in the school's music room and collapsed as a result of the leak.

"I received a call from the teacher of my daughter who said she was not feeling well and had been vomiting and weeping. I was the first person who came here before the emergency crews. I took her home and I asked her what happened and she told me she fell down along with a number of other kids," said Muhammad Baber.

Baber says his other child also goes to the school and is in good health.

He says the teacher told him something happened in the music room but he isn't exactly sure what occurred.

Fire officials have since announced the all clear at the school and say that no dangerous gas or chemicals of any kind were found in the school.

The school board, meanwhile, says the building is safe for students to return.

"The school has been cleared by the fire department," says Megan Geyer with the Calgary Board of Education. "So we're confident the school is safe for students to return."

Geyer says that parents are free to come and pick their children up from the school if they feel that's something they need to do.