Amid the Japanese authorities' investigation into a bribery scandal related to last year’s Tokyo Olympics and Paralympics, further allegations of bid-rigging by the nation’s leading advertising firms for Games test events have surfaced.

Over the past week or so, investigators from the Tokyo District Public Prosecutor's Office and the Fair Trade Commission have raided several firms including Dentsu, Hakuhodo and ADK Holdings — Japan’s top three advertising companies — over the case.

The new development came after the arrests of a former executive of the Tokyo Organising Committee and some other prominent business figures over alleged bribery.