The investigation comes after allegations made about the club's ownership in line with existing regulations.
Current policies state that the ultimate beneficial owners of a Football League club must be made known.
A League statement read: "Having taken advice from a League counsel in regard to existing league regulations, it was unanimously agreed that it will be the policy of the board to insist that it is informed, with supporting evidence provided, of the ultimate beneficial owners of all Football League clubs.
"The board noted recent allegations made about the ownership of Leeds United Football Club. The League has written to the club seeking clarification. No further comment will be made on this matter until a response has been received."