CHICAGO (CBS)– A carbon monoxide leak at an elementary school on the South Side prompted a level 2 hazardous materials response Monday morning, and led to the school moving classes to another school for the rest of the day.

A custodian working early Monday morning at Burnside Academy discovered the carbon monoxide detector going off.

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No children were in the building at the time of the incident. Fire crews responded to the school and secured the scene.

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Officials believe the school’s boiler system was the cause of the leak, and repairs to the boiler are expected to take all day.

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Chicago Public Schools said students were transported to BUrnside Church at the start of the school day, and later were bused to Dixon Elementary School. The students will be brought back to Burnside for pickup at the end of the day.