Japanese started offensive in the Indian Ocean according to the U.S. Joint Intelligence Committee. Daily summary dated 2 April 1942

An item reported that the Japanese started their major offensive in the Indian Ocean. The merchant shipping in the Bay of Bengal especially at Calcutta would be the first victim. Except for aircraft the Japanese forces possible included 5 heavy cruisers, 10 destroyers and 12 submarines. A British tanker was on 29 March twice bombed about 200 miles south of the entrance to the river Ganges.

Source

Map Room Papers (Roosevelt Administration), 1942 - 1945. U.S. Joint Intelligence Committee. Daily summary No. 113 dated 2 April 1942