SEOUL, South Korea — Japan's largest advertising agency Dentsu and five other firms have been indicted for alleged bid-rigging in the run-up to the 2021 summer Olympics. The deepening scandal suggests that preparations for some of the world's highest-level sporting competitions were anything but competitive.

Prosecutors issued the indictments after receiving complaints from Japan's Fair Trade Commission. The complaints say that Dentsu, its main rival Hakuhodo, and four other firms and seven individuals rigged bids for Olympic test events.

The events were dress rehearsals held between 2018 and 2021 to test Olympic venues, and familiarize athletes and staff with them. The games will largely be remembered for being delayed by a year, and being held despite widespread public opposition to going ahead with the games during the COVID-19 pandemic.

Dentsu Group President and CEO Hiroshi Igarashi admitted to prosecutors his firm's involvement in the bid rigging, Japanese media report. About half of the 26 test events had only one firm bidding for each, resulting in more than $300 million worth of contracts being awarded without any competition, a possible violation of Japan's antitrust law.

Dentsu was in charge of arranging corporate sponsors for the games, a role it has been involved in since the 1984 Los Angeles Olympics.

Prosecutors arrested a former Dentsu executive last year in a separate Olympic corruption probe. Haruyuki Takahashi, a former Tokyo Olympic Organizing Committee member, was detained along with the heads of several firms suspected of bribing him in exchange for Olympic sponsorship deals.

French prosecutors have also investigated Takahashi, on suspicion that he bribed a member of the International Olympic Committee (IOC), in a bid to secure Tokyo's right to host the games.

Tokyo Governor Yuriko Koike says that if the test event bid-rigging allegations are proven, she will seek damages from Dentsu and other organizers, for driving up the costs of hosting the games for host city Tokyo, and for taxpayers.

One possible casualty of the corruption scandals is the northern Japanese city of Sapporo. Sapporo is the front-runner among possible hosts of the 2030 Winter Games. But it suspended promotion of its bid in December, amid public outrage at the corruption scandals. The IOC has postponed selecting a host for the 2030 games, amid concerns about climate change.