Russian rescuers are still searching rivers, quarries and reservoirs for an Antonov An-2 transport missing for days after departing unauthorised from Serov, about 300km north of Ekaterinburg.
The sturdy biplane took off without clearance from Serov at around 23:00 local time on 11 June.
Six aircraft have been attempting to sight the single-engined An-2. Efforts to locate it have focused on an area within a 50km radius of Serov.
Russia's emergencies ministry has appealed to anyone who saw the aircraft on that day, or the following one, to contact its officials.
It has also sought volunteers from Serov and local towns to the northwest - Karpinsk, Krasnoturyinsk and Severouralsk - to hunt for the An-2 which, the ministry says, had a pilot and 12 other people on board.
The ministry says it is looking for hunters and outdoor sports enthusiasts to search mountainous areas - such as Konzhokovsk Kamen in the west - particularly in areas where vehicles are unable to travel.
Divers have been recruited to examine quarries and city reservoirs, and vessels have been checking the Kakva river.
The Ural transport investigation division says the aircraft took off "without notification to air traffic control" and has opened a criminal probe into the event, assuming a fatal air transport accident.
It says: "The second pilot and mechanic found the aircraft gone after they returned from a short visit to the town of Serov."
Investigators have examined vehicles at the scene, it adds. Preliminary information points to the aircraft having been leased for monitoring of forest fires in the region.