MOSCOW (AP) Russian investigators are searching the state aviation agency to see if some pilots may have fake licenses part of an investigation into a plane crash last month that killed all 50 people on board.

Investigative Committee spokesman Vladimir Markin said in a statement Friday that investigators believe some pilots working for Russian airlines have received fake licenses in centers certified by the aviation agency.

Markin said there were reasons to believe that the pilot who sent the Boeing 737 into a near-vertical dive after an aborted first landing attempt had received his license illegally in a small training center that was no longer functioning.

No criminal charges have been filed yet in the crash, which took place in the city of Kazan, 720 kilometers (450 miles) east of Moscow.