BOONE, N.C. -

There are new developments in the investigation into the carbon monoxide poisoning deaths of three people at a Boone, North Carolina hotel.

We've learned the owner's manual for the pool heater at the Best Western Plus Blue Ridge Plaza Motel warns detectors for the odorless gas should be put in when the heater is installed.

The front page of the manual also has a warning strongly recommending carbon monoxide detectors and inside, warns improperly installing the heater could cause death from carbon monoxide gas.

Boone authorities say the motel did not get a permit before installing the used heater in 2011.

A married couple at the hotel in May and an 11-year-old boy died in June in the room above the pool heater. Police say they died of carbon monoxide poisoning. Click here to read more.

Police are still investigating the deaths.