Twenty-one people on houseboats for a family gathering at Lake Powell were treated for carbon-monoxide poisoning, just a few days after a 7-year-old girl drowned when she was overcome by carbon monoxide fumes, officials said.

The odorless gas came from exhaust from generators that were used to run air conditioners, said Marianne Karraker, spokeswoman for Glen Canyon National Recreation Area on the Utah-Arizona border.