Investigators in Russia are examining whether the pilot of a passenger jet which crashed near Kazan with 50 deaths had a valid pilot's licence.
The Boeing 737's captain may have only been qualified to serve as a navigator, the Investigative Committee said.
It appears he obtained his commercial pilot's licence from a flight centre which has since been closed down.
Searches have been carried out in offices of Russia's Federal Air Transport Agency (Rosaviatsia).
All passengers and crew were killed when the Tatarstan Airlines plane crashed at Kazan airport on 17 November, while trying to land.
Russian aviation experts said soon afterwards that the pilot, Rustem Salikhov, had carried out a manoeuvre which put the plane into a dive.
During a second attempt to land, the crew had switched to manual control and made a correction when the plane lost speed, the investigators said.
The jet then nose-dived on to the tarmac.
According to initial findings, the jet did not have any technical faults.
On Friday, Investigative Committee spokesman Vladimir Markin said that investigators were still examining Salikhov's qualifications.
"To date it has been established that the plane's captain, Rustem Salikhov, was trained as a navigator and then apparently obtained a commercial airline pilot's licence in a flight centre licensed by Rosaviatsia," he said in a statement.
Investigators are examining the legality of the work of these flight centres, which have now been shut down, he added.
Documentation on their work was being sought from Rosaviatsia, he said.
Mr Markin told Russia's Interfax news agency there was reason to believe "many pilots, working for smaller Russian airlines" had "effectively received fake commercial licences" as they had not undergone proper training.