GREELEY, Colo. (AP) — A faulty boiler used to heat the swimming pool at a recreation center in northern Colorado is believed to be the source of carbon monoxide that sickened 12 people, including at least one child who was hospitalized.

The Greeley Tribune reports that they were exposed to the deadly odorless gas Monday night during an archery practice in the Greeley Recreation Center’s basement.

The wife of an archery club leader, Jim Flores, didn’t feel well and went out to get some fresh air. When she returned, she noticed she barely had the strength to climb stairs. Around the same time, others complained of headaches and nausea, so Flores asked everyone to evacuate.

The Flores’ son, who suffers from asthma, was admitted to the hospital. The others were treated and released.