FILE PHOTO: A BlackRock Inc sign above its building in New York, United States, July 16, 2018. REUTERS/Lucas Jackson/File Photo

DUESSELDORF, Germany (Reuters) - Friedrich Merz, chairman of the German arm of BlackRock BLK.N, the world's biggest fund manager, said he had ordered the fund's management board to fully investigate all incidents involving so-called cum ex transactions and cooperate with prosecutors.

Merz spoke at a political event in the northwestern German city of Duesseldorf, hours after a source familiar with the matter said prosecutors had searched BlackRock’s Munich offices as part of Germany’s largest postwar fraud investigation.

“I have ordered the management board to cooperate with the investigative authorities and put all documents on the table. Everything will be investigated and we will actively participate in that,” said Merz, a candidate to succeed German Chancellor Angela Merkel as leader of the Christian Democrats (CDU).