A carbon monoxide leak at a Michigan hotel Saturday morning led to one child's death and a dozen people, including six other children, to be rushed to the hospital, police said.

The child died en route to the hospital, while the other children were expected to make a full recovery, a spokesperson for the Lakeland Health Hospital in Niles told NBC News. They range from ages 10 to 14.

Two hotel workers at the Quality Inn and Suites in Niles, along with two Berrien County officers, one Nile city police officer and another adult were also treated for exposure to the poisonous gas. Their conditions were not immediately known.

Hotel staff found the children lying unresponsive at an indoor pool deck at 10 a.m. ET after noticing their figures through a window, Niles Fire Capt. Don Wise told NBC affiliate WNDU.

The carbon monoxide monitor used upon entering the hotel revealed that the levels were 16 times the safety limit at 800 parts per million, Wise said. Normally, the safety code permits 50 parts per million.

"When we first went it, all the monitors went off," Wise told WNDU. "The time it would have taken for us to go back out and get our air packs in, we grabbed the patients and started pulling them out."

"I have never seen something like this in 30 years," the fire captain added.

One witness told WNDU that one of the mothers was screaming in the hallway when she came upon the children's limp bodies.

"She was screaming, 'My kids, my kids,'" the witness said. "I didn't see any movement in their bodies at all."

Related: 'Very Tragic': Carbon Monoxide Blamed for Killing Six in Michigan Family

Niles authorities said it was too early to know the cause of the carbon monoxide leak, but Wise suspects it could have been from a natural gas heater.

"Carbon monoxide is odorless, colorless, tasteless. You'll wind up getting flu-like symptoms, vomit, then go unresponsive," he added, "but there isn't much time between getting nauseous and going fully unconscious."

The hotel was evacuated and remained shut down after the incident.

The local health department planned to inspect the facility Saturday afternoon to determine if the pool should remain closed and the rest of the hotel could be reopened.

Wise said that the number of victims could escalate as some of the first responders are currently experiencing headaches, one of the first signs of poisoning.