The plane owned by the Moskovia air carrier was en route from Chelyabinsk to Perm when it crashed to the north of Chelyabinsk at 6:30 p.m. Moscow time (14:30 GMT).
None of the bodies have yet been identified.
Alexander Neradko, the head of the Federal Air Navigation Service, said the investigation into the accident could continue for more than a month.
The emergencies ministry said on Monday that an air traffic controller had received a message from the cargo plane's crew shortly after takeoff from Chelyabinsk airport, saying smoke was entering the pilot's cabin and that the crew had decided to return to the airport for an emergency landing. The plane then disappeared from the radar screens.
Vyacheslav Latunkin, a spokesman for the local department of the emergencies ministry, said the plane most likely hit an electric power line during the emergency landing. The plane's black box was found yesterday, and will soon be decoded.
However, Neradko said the crash could have been caused by a short circuit.
The Kremlin said yesterday President Dmitry Medvedev had expressed his condolences to the families of those killed, and ordered Transportation Minister Igor Levitin to set up a commission to investigate the tragedy, and provide help to the relatives.
The last major accident involving an An-12 cargo plane occurred last July, when a plane crashed near Moscow's Domodedovo airport soon after take-off, killing all seven crewmembers. As the investigation later showed, the crash was caused by birds who got trapped in the engines on take-off, stalling two of the four engines.
