Twelve people died on Tuesday when a night time fire tore through a two-story Moscow market warehouse that was being used by migrant workers from a former Soviet republic as a temporary residence.
"Twelve people died. They were migrant workers. We are trying to confirm which (ex-Soviet) republic they came from," a Moscow emergencies ministry spokesman told.
Unconfirmed news reports said the migrants came from the impoverished Central Asian nation of Tajikistan. Numerous Moscow markets employ cheap labourers from the region without giving them proper housing conditions or pay.
The blaze erupted early on Tuesday at a market on the southern outskirts of the city called Kachalovsky. Officials said it took two hours to put out. Emergency workers described squalid living conditions in which the workers slept on hard cots that were stacked on top of each other in rows of four without any direct access to the outside.
The workers "lived in a metal annex that was equipped with a space heater", an unnamed law enforcement official told the Interfax news agency.
"They slept in frighteningly tight conditions, on hard bunk cots that were then stacked on top of each other," he said.
Another official said the workers probably left the space heater on all night to stay warm during the frigid Moscow spring. Overnight temperatures plunged below freezing and much of central Russia has been hit by snow.
"The fire could have started from the electric space heater. It was probable left on all night because the structure was turned into living quarters," an unnamed investigator told ITAR-TASS.
Police said they have opened a criminal investigation but have not yet pressed any charges against the market owners.