Officials in eastern Ukraine are investigating the second accident to hit one of the country's coal mines in as many weeks. Six miners died and at least 14 were injured in the underground methane gas explosion on Sunday.
Ukrainian investigators are focusing on the possibility the accident may have been caused by blasting operations in and around the mine.
The investigation is in its first hours, as it took most of Sunday to put out the fire and to rescue the more than 400 miners working underground at the time of the explosion.
The mine is located in Dniepropetrovsk in eastern Ukraine. Officials there have declared Tuesday a day of mourning in order to bury the dead.
At least 14 miners remain hospitalized - two of them reported in serious condition.
The explosion occurred exactly two weeks after a fire raged through a coal mine in the region of Donetsk, killing 35 miners.
An official investigation into that incident revealed the fire was caused by a spark on a conveyor belt. Three managers have since been arrested for alleged violation of security regulations.
Experts say Ukraine's mines have been plagued by inadequate funding, coupled with a lax regard for safety rules. In the first seven months of this year, more than 120 Ukrainian miners have died on the job. Those figures, and a months-long delay in pay, prompted the miners to protest late last week.