They also found a downed helicopter.

The bizarre turn of events added a peculiar twist to the mystery of what happened to an ageing Antonov An-2 biplane that took off without permission from a disused airfield near the Ural Mountains steel mill town of Serov.

State television crews have since travelled to Serov to interview locals who have remained almost unanimously tight-lipped about what happened on the weedy air strip on the evening of June 11.

Police have already probed theories of the An-2 being taken over by some friends who decided to go on a fishing trip after a long weekend party and the plane being used in some ill-conceived recreational flight scheme.

The An-2 hunt is still being officially called a "search and rescue" mission because there is no confirmation of the plane going down -- and no recorded radar evidence of it ever having taken off from the airport.

But the region's emergencies ministry said Thursday it did find the remains of a Soviet-era An-2 that crashed in 2006.

It also noted spotting the remains of a helicopter at the same location. The two had apparently crashed but officials reported no details.

"The findings have no relation to the missing An-2 whatsoever," the regional ministry announced.

Russian media speculate that up to 12 people may have been on board the single-engined propellor-driven An-2 when it took off from Serov.

State television has further suggested that city authorities had ordered instructions for the locals not to cooperate with the investigation out of fear that they might become implicated in unspecified crimes.

Rescuers have reported scouring some 220,000 square kilometres (85,000 square miles) of land from the air and another 1,000 square kilometres by foot without finding anything but the two old wrecks.