Olathe, KS – KMBC-TV -Twelve people were treated for carbon-monoxide poisoning early Monday morning.

Emergency crews were called to a duplex in the 200 block of South Lake. Firefighters evacuated both sides of the building.

Investigators said a disconnected furnace flue pipe in the attic was the problem.

Firefighters said the carbon monoxide levels were five to 10 times above the danger level.

“We had high levels of carbon monoxide on both sides. Our meters peak out at 999, and they peaked out at that level. So we knew we had significant amounts of carbon monoxide inside,” Battalion Chief Ken Keiter said.

Eight people remain hospitalized. Some are in serious condition.

The people who lived in the second part of the duplex were evacuated, but were allowed back in a short while later.