MARYSVILLE, Ohio (WCMH) — Investigators are still working to determine the cause of a carbon monoxide leak that left more than a dozen central Ohio hotel guests hospitalized Saturday evening.

It happened at the Hampton Inn in Marysville shortly before 5:30 p.m.

“Very quickly. Very quickly,” warns Marysville Fire Chief Jay Riley. “It can be. Yeah, in this case, it was a matter of minutes.”

That’s how fast Chief Riley says exposure to carbon monoxide can become life-threatening.

Emergency crews were called the Hampton Inn on Square Drive after receiving reports of an unconscious 2-year-old girl.

“We really didn’t know what we were getting into. We thought we had just one child that was unresponsive in the pool, from there we had other victims identify themselves,” Chief Riley details.

But as dispatchers received more calls, they began to learn that the little girl wasn’t alone.

“We ramped up our response, we had additional medic units respond,” explains Chief Riley.

More than a dozen guests in the hotel’s pool area — including six children — ended up hospitalized after investigators discovered high levels of carbon monoxide throughout the hotel.

“They had an excessive amount of carbon monoxide in a short amount of time,” Chief Riley adds.

The hotel is privately owned and operated by a company called the “Amerilodge Group” out of Michigan. In a statement, a spokesperson for the company told NBC4 they are fully cooperating with local authorities.

As of Saturday evening, as spokesperson with Memorial Hospital told NBC4 of the 11 patients they saw, two were critical but stabilized, five were in serious condition but stable, and four others were treated and released.

We’re told three other victims were taken to Grady Hospital, while two others were treated at the scene.

On Sunday, that same spokesperson told NBC4 the patients had been moved to tertiary care facilities to continue their recovery, and no other updates on their condition would be available.

The hotel was evacuated immediately and remained closed to guests on Sunday.

Officials say crews have been busy ventilating the building and working to find the source of Saturday night’s leak.

And while investigators spend the coming days searching for a cause, they encourage people to take safety measures in their own homes as well.

“It’s always important to have a carbon monoxide detector in your home, make sure it’s tested,” encourages Chief Riley.

Chief Riley says the police and fire departments are working jointly on the investigation. They expect the hotel should be safe to re-open in the next day or two.