WATERBURY, Conn. -- Students at Barnard Elementary School in Waterbury were sent to local hospitals after a carbon monoxide leak on Tuesday afternoon.

Officials said at least one student passed out inside the school and others began to complain about nausea and dizziness.

"We had a kid pass out," said Dr. Peter Jacoby, of St. Mary's Hospital. "The fire department responded on a 911 call and realized there was something else going on."

Officials at Waterbury Hospital said 12 students and a teacher were treated and released from the hospital. Officials at St. Mary's Hospital said 17 students were treated there.

All of the people who were sent to local hospitals were expected to be released by Tuesday evening, according to hospital officials.

The superintendent of school said the incident was caused by an open door leading from the furnace to a chimney. That door is now sealed.