Arch Coal spokeswoman Kim Link confirmed for International Longwall News Monday that the operation was cleared of its workers August 23 at about 3pm local time when the monitoring system detected "slightly elevated" carbon monoxide levels.
"We elected to evacuate the mine while we investigated the potential source of those readings," Link said.
"The decision to evacuate the mine was precautionary; we are always exceptionally cautious on any issue that involves worker safety."
Following the federally regulated 72-hour waiting period, she said officials re-entered the mine Sunday.
"[We] investigated the area that was the source of the heightened readings and we found no evidence of combustion," she added.
US Mine Safety and Health Administration representative Rick Manning spoke to the contrary late last week to local newspaper the Salt Lake Tribune, calling the incident a "gob fire".
Manning reportedly said the agency sent eight experts to the operation's location with mine rescue teams also onsite, but that "there were no injuries, everyone got out okay".
Dugout Canyon mines from the Book Cliffs area of Carbon County, Utah, about 12 miles north of Wellington.