The antitrust watchdog Monday searched Rakuten Inc.’s head office in Tokyo over the internet mall operator’s plan to make marketplace tenants shoulder the cost of its proposed free shipping service, sources close to the matter said.

The Fair Trade Commission last month launched an investigation on suspicion the e-commerce giant is breaching the anti-monopoly law by abusing the company’s dominant position in relation to smaller online retailers that depend on its platform, the sources said.

The probe came after a group of around 450 merchants filed a petition to the commission in January claiming that Rakuten is forcing them to bear the costs of shipping for all orders exceeding ¥3,980 starting March 18.

While Rakuten President Hiroshi Mikitani has said the company will go ahead with the plan, the company said in a statement issued last week that it will fully cooperate with the probe.

At present, the roughly 49,500 merchants on Rakuten’s online marketplace set shipping fees for items independently, with some providing free shipping.