By Nancy Coleman

CNN

ATLANTA (CNN) -- Fire crews responding to a call about an unconscious child at a birthday party soon found themselves dealing with a much bigger emergency: Nine people, most of them children, sick or unconscious with signs of carbon monoxide poisoning.

The incident happened Tuesday in Sharpsburg, Georgia, just south of Atlanta, as the kids played in a rented video-game trailer. A leak from the trailer's generator filled the trailer with dangerously high levels of carbon monoxide, authorities said.

It started with just one child passing out -- but when another stumbled and fell unconscious, parents began to evacuate the trailer.

GameTruck, the company that rented out the vehicle, said it has "never had an incident like this in the 11-year-history of the company." In a statement sent to CNN, the company said it was working on replacing carbon monoxide detectors in all trailers "immediately."

Nine children and two GameTruck employees were in the trailer for about 90 minutes before the symptoms began, according to the incident report from Coweta County Fire and Rescue. Both employees and seven of the children were taken to local hospitals; everyone was treated and released the same day.

The children ranged in age from 6 to 11 and the employees were 18 and 20, Fire Capt. Craig Sherrer told CNN. Parents were inside the house watching the younger kids. The owner of the home where the truck was parked could not be reached for comment.

After inspecting the trailer, firefighters measured 27 parts per million of carbon monoxide inside -- 2 ppm more than what Sherrer said people should be exposed to over an 8-hour period.

He said the firefighters "know for a fact" that the levels were much higher than that when the children were still inside, since the generator had been turned off and measurements were taken 45 minutes after emergency crews arrived.

Firefighters said the carbon monoxide detector inside the trailer was not working. They noted in the incident report that the detector was "chirping as if the battery were dead."

GameTruck operates franchises in more than 30 states, offering climate-controlled trailers filled with video games, game consoles and TV screens.

In its statement, GameTruck said it requires routine maintenance for all equipment.

"We are currently looking into the situation and hope to identify the cause of the problem in the coming days," the statement said.

Sherrer said children and elderly adults are more prone to carbon monoxide poisoning, which is difficult to sense because the gas is colorless and tasteless. The first line of defense, he said, is a carbon monoxide detector.

Typical symptoms include headaches, fatigue and weakness. Sherrer also cited a "group of people randomly falling unconscious" as a warning sign.