FORT WAINWRIGHT, Alaska (AP) — Most of the Alaska-based paratroopers who were hospitalized after being exposed to carbon monoxide have returned to duty.

U.S. Army Alaska says in a release that 17 of the soldiers were treated and released after the Thursday incident at the Yukon Training Area near Eielson Air Force Base in Fairbanks. They were to be returned to their unit Friday at Joint Base Elmendorf-Richardson in Anchorage.

Four other soldiers were admitted to the Fort Wainwright base hospital. U.S. Army Alaska spokesman John Pennell says they are in good condition, and should be released to their unit at Joint Base Elmendorf-Richardson in Anchorage on Saturday.

None of the injuries was life-threatening.

Pennell says the cause of the carbon monoxide exposure, which occurred Thursday during a field training exercise, remains under investigation.

Copyright 2014 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.