Investigative Committee spokesman Vladimir Markin said that the investigators believe some pilots working for small regional Russian airlines aren't properly trained but have received fake licenses in centers certified by the aviation agency.
Markin said that the pilot who sent the Boeing 737 into a dive after an aborted first landing attempt had received his license in a small training center that has since been shut down. He said the investigators were conducting searches for documents related to the activities of such centers.
No criminal charges have been filed yet in the Nov. 17 crash of the Tatarstan Airlines jet in the city of Kazan, 720 kilometers (450 miles) east of Moscow.
Russia's federal aviation regulator, Rosaviatsiya, said it is investigating to check the legitimacy of pilots' licenses that were issued at the training center mentioned by Markin and other similar facilities.
The agency head, Alexander Neradko, said it was checking all pilots who had been trained in such centers.
Neradko said they had been created to help fill the shortage of pilots, and acknowledged that they were later shut because of flaws.
The Moscow-based Interstate Aviation Committee, which investigates plane crashes across the former Soviet Union, have previously concluded that the crew failed to land at first attempt, began to stall in a steep climb, then fatally overcompensated.
The report was based on the data retrieved from the plane's flight parameters recorder, which also showed that its engines and other systems were working fine until the plane hit the ground.
Investigators are yet to release the data retrieved from a cockpit conversations recorder, which could help shed light upon the reasons behind the series of faulty maneuvers that led to the crash.