Tokyo prosecutors are learning that leading Japanese ad agency Dentsu played a larger role in a bid-rigging scandal involving the Tokyo Olympics and Paralympics.

Sources have told NHK that Dentsu is suspected of advising the Tokyo Olympics and Paralympics Organizing Committee on how to write a document about specifications with detailed terms and conditions for bidders. Dentsu was a bidder and was awarded some projects.

According to the sources, in the autumn of 2017, the year before the bidding took place, Henmi Koji, a former senior Dentsu official, set up an in-house team to support the organizing committee. Henmi was said to be virtually leading the team.

Mori Yasuo, who was the deputy director of the committee's operations bureau at the time, had requested such a team to Henmi.

Previous investigations had revealed that Dentsu provided Mori with its research results on the past tendering records of each bidder.

Dentsu is also suspected of developing for the committee an assignment list of bidders to specific sports.

Tokyo prosecutors are believed to be investigating the case, suspecting that Dentsu, which participated in the bidding, and the organizing committee, which placed the order, worked together to manage the bidding.

They have arrested four people, including Henmi and Mori, in connection with the bid-rigging for Games projects worth 40 billion yen, or about 304 million dollars.

The biddings covered work to develop plans for test events and the operation of main events.

The four are suspected of violating Japan's Anti-Monopoly Act.