
The document served as the financial guarantees from the Government for the successful bid, which the South Korean city was awarded last month.
Kang has denied that he was responsible for forging the signatures and has blamed a low-ranking official.
Prosecutors also raided the residence of Kim Yoon-suk, the event's Bid Committee secretary general.
Kang and Kim are both expected to be questioned formally later this month.

The scandal was uncovered on the eve of the vote but world governing body FINA still awarded the Championships to Gwangju, the first time they have been given to South Korea.
The affair is hugely embarrassing for Kang, who has adopted a high-profile role as Gwangju has moved into the international spotlight.
The city, the sixth largest in South Korea, is also due to host the 2015 Summer Universiade.
Gwangju estimated the South Korean Government's financial support for the World Aquatics Championships at 5.5 billion won (£3 million/$5 million/€3.8 million).
The Government has insisted that following the scandal it will spend nothing on the event.
FINA has so far not commented on the matter.
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