Prosecutors expanded their investigation into the owner of the sunken South Korean passenger ferry on Wednesday as inspectors confirmed that the cargo weight declared by the ship on its final voyage was more than three times the recommended maximum.

Officials from Chonghaejin Marine Co., the operator of the ferry, were banned from leaving the country as investigators searched the offices of the company and its affiliates, as well as the residence of the firm's owner, Yoo Byung-eun, and other companies he owns, prosecutors...