Russian investigators are combing through the charred fragments of a Boeing 737 jetliner as they try to determine what caused its crash that killed all 50 people on board.

The plane belonging to the Tatarstan Airlines crashed Sunday while trying to land at its home port in the Russian city of Kazan, the capital of the oil-rich province of Tatarstan. The son of the provincial governor was among the victims.

Alexander Poltinin, the head of the local branch of Russia's Investigative Committee, said Monday the plane crashed while making a second attempt at landing. He said the investigators are trying to determine why the crew couldn't land the first time.

Poltinin said the investigators are looking into a possible pilot error or an equipment failure.