Massey Energy has received a citation for improperly stored explosives at the Upper Big Branch mine in West Virginia, US, where 29 men were killed in an explosion in April.
Massey said the US Mine Safety and Health Administration (MSHA) issued an imminent danger order after a box of explosives was found at the on 30 July.
The explosives were found between two areas involved in the explosion, reports The Associated Press.
The explosion is being investigated by federal officials.
Last week the MSHA released documents indicating that it has cited Massey for failing to report more than 20 accidents at the mine in the last two years.