Tokyo prosecutors raided the headquarters of ad agency Dentsu Inc and an events company on Friday, part of a widening scandal into corruption during the Tokyo 2020 Olympics.

The companies are being investigated on suspicion of rigging bids for contracts related to Olympics test events, public broadcaster NHK and other domestic media reported.

The new probe follows a sponsorship bribery scandal that also saw Dentsu Inc offices raided earlier this year.

Shares in Dentsu Group Inc, the parent company of Dentsu Inc, finished 2.7 percent lower on the news of the fresh investigation, while shares in the event company Cerespo Co Ltd, which media also said was raided, tumbled 11 percent.

According to the Kyodo news agency, the raid was conducted after advertising agency ADK Holdings informed the Fair Trade Commission of its involvement in collusion over bid rigging.

In the earlier bribery scandal, Haruyuki Takahashi, a former member of the Tokyo 2020 Olympics board and before that a Dentsu Inc executive, was arrested in August on suspicion of receiving bribes from Olympic sponsors.

Executives from ADK, fashion retailer Aoki Holdings and publishing firm Kadokawa have also been arrested in connection to the bribery scandal.

Dentsu Inc. is part of the domestic arm of advertising behemoth Dentsu Group Inc. The group, valued at some USD 9 billion, also has an extensive international business.