Firefighters were called to the warehouse around 2:15 p.m. when the owner of the building found the three inside, Koby Johns, public information officer for the department, told the Fresno Bee ( http://bit.ly/12uZdW8).
The three people were working and living in the warehouse with the owner's permission, officials said.
The generator was being used for power after the electricity to the 3,000-square-foot building had been shut off two days earlier, Johns said.
Safe levels for carbon monoxide are considered to be 35 to 50 parts per million, but levels were measured in the warehouse at more than 100 parts per million after an hour of ventilation, Johns said.
When coroner's investigators arrived at the warehouse they were not able to enter the building for two hours because the carbon monoxide levels remained high.
The names of the three people—believed to be in their 40s—have not been released.
The incident comes just a week after fire officials gave away 50 carbon monoxide monitors to local residents during an event being held about 200 feet north of the warehouse, Johns said.
Information from: The Fresno Bee, http://www.fresnobee.com