The pilot of an airliner that crashed near the Russian city of Kazan last month, killing 50 people, may have received his licence from a training centre that was later closed on suspicion of operating illegally.
The November 17 crash killed all 44 passengers and six crew, and highlighted the poor safety record of regional airlines that ply internal routes across the world's largest nation.
Crash investigators said at the time the pilot of the Tatarstan Airlines Boeing 737 had aborted a first attempt to land.
In making a second effort, he had pushed the steering column forward, pitching the plane into a nosedive that brought the aircraft crashing into the tarmac.
Investigative Committee spokesman Vladimir Markin told Interfax there was reason to believe that many pilots working for smaller Russian airlines had effectively "received fake commercial licences" as they had not undergone proper training.
The pilot of the Tatarstan Airlines jet is believed to have been issued his commercial pilot's licence by a training centre that was accredited by airline regulator Rosaviation
"Investigators continue to look at the circumstances in which the crew of the crashed jetliner were trained," Markin said in a statement. "The investigators have doubts about the legality of the activity of the said aviation training centre, which was liquidated."
The Investigative Committee was carrying out searches in some of Rosaviation's departments and plans to question managers at the flight regulator.
The head of Tatarstan Airlines, Aksan Giniyatullin, was fired over the crash, the province's ministry of transportation said.
A Rosaviation committee recommended grounding Tatarstan Airlines after a post-crash inspection revealed that some of its personnel were overworked or inadequately trained.
Analysis of accident statistics shows that flying as a commercial airline passenger in Russia is roughly four times as dangerous as the world average.
Worldwide, on average, one passenger dies for every 4.7 million that board a commercial airliner.
In Russia, that ratio is one to 1.2 million, according to figures from international airline group IATA, the Aviation Safety Network and Rosaviation.