Authorities are trying to find the source of carbon monoxide at an Ohio hotel that sent at least 11 people, more than half of them children, to a hospital.

Marysville Fire Chief Jay Riley told the Columbus Dispatch that everyone who was hospitalized had been in the pool area of the Hampton Inn.

A hospital official said victims had been stabilized or upgraded. Riley said Sunday morning that he had no immediate update on the investigation.

A representative of the Hampton Inn Marysville said Sunday that officials were “fully cooperating with the local authorities as they investigate this incident.”