Updated: As of Friday morning, 17 of the soldiers have returned to their unit. The four soldiers who were hospitalized Thursday remain at Bassett Army Community Hospital and are expected to be released Saturday, according to Army spokesman John Pennell. The cause of the carbon monoxide poisoning remains under investigation. 

FAIRBANKS - Nearly two dozen Anchorage-based soldiers were treated in Fairbanks on Thursday for carbon monoxide exposure they sustained during a morning field training exercise.

The soldiers, paratroopers from the 4th Infantry Brigade Combat Team (Airborne), 25th Infantry Division at Joint Base Elmendorf-Richardson, were in a multi-day exercise at the Yukon Training Area near Eielson Air Force Base when they began to show symptoms, according to an Army news release.

Carbon monoxide is a colorless, odorless gas produced by combustion that causes headaches, dizziness and potentially loss of consciousness and death in cases of prolonged exposure.

None of Thursday's cases appear to be life threatening, said Army spokesman Lt. Col. Alan Brown. There's no reason to believe additional soldiers, including Fairbanks-based soldiers, sustained carbon monoxide poisoning, he said.

The Army is still investigating the cause of the exposure.

"It's too early in the investigation to be able to say definitively, so I wouldn't want to speculate at this time," Brown said.

The soldiers were driven to Bassett Army Community Hospital at about 9:30 a.m.

Among the 21 soldiers who showed symptoms, four were admitted to the hospital and seven were released.

Eight patients are awaiting lab results and two were still being evaluated Thursday evening. The soldiers who were released from the hospital will spend the night at Fort Wainwright and will rejoin their units Friday, Brown said.

Contact outdoors editor Sam Friedman at 459-7545. Follow him on Twitter: @FDNMcrime.