An accident at the underground mine in Estonia caused the death of two workers late on Thursday.

ERR's correspondent in the area, Ago Gaškov, said the bodies were found by the next shift. No signs of violence or a cave-in were found.

"One theory is a gas or carbon monoxide poisoning. There might have been problems with the ventilation," Gaškov said.

Both men were in their late 30s and had worked in mines for 18 years each. The men were tasked with digging new paths underground.

The "Estonia" mine is the nation's largest underground oil shale rock mine. It is located in Ida-Viru County and was opened in 1972.

The incident is the second major one to hit Eesti Energia this month. On January 6, tons of ash fell on a group of workers at the company's Auvere Power Plant. Eight were injured, three seriously.