Fire officials said they believe the cause was carbon monoxide poisoning.
The 14 were taken to local hospitals Saturday, according to police in Niles, a town of about 11,000 just north of the Indiana state line.
Jessica Hines, a spokeswoman for Lakeland Hospital there, told CNN one person was "confirmed dead on arrival."
The Niles Police Department identified the deceased as 13-year-old Bryan Douglas Watts of Niles.
Eight other children were taken to Memorial Hospital in South Bend, Indiana, police said. Three were released by Saturday evening and the other five were reported in good condition Sunday at noon.
They include five children found unconscious alongside Watts at the Quality Inn & Suites hotel pool. Their ages range from 12 to 14 years old, Niles police said.
Another child, who police said had just left the pool area, was found unresponsive in a first-floor room.
Five first responders were also treated and released.
Carbon monoxide levels were high, police say
Police responded to the hotel around 10 a.m. ET, according to CNN affiliate WNDU-TV
Police said that, when they arrived, carbon monoxide levels at the hotel were 800 parts per million, according to CNN affiliate WBND-TV
. US standards for carbon monoxide are 35 parts per million
for a one-hour exposure.
Niles Fire Department Captain Don Wise attributed the incident to a faulty pool heater.
"The ventilation system on the heater had issues. The exhaust was not functioning properly," said Wise. "Our mechanical inspector verified that. The hotel is shut down now until repairs are made."
He said the hotel's pool room had no carbon monoxide detectors "to the best of our knowledge."
Carbon monoxide is an odorless, colorless and tasteless gas
that is produced when a fuel is burned. When too much carbon monoxide is in the air, it can replace the oxygen in red blood cells and can lead to tissue damage or death.
Choice Hotels, which owns the Quality brand, released a statement about the incident, saying it is "working closely with local officials to manage the situation. ... Our highest priority is always the safety and well-being of our guests."