A Japanese contractor is being investigated by U.S. authorities for possibly dumping wastewater from U.S. warships into Japanese harbors, U.S. media reported Sunday.

Kanto Kosan Co. is suspected of disposing wastewater at ports in Yokosuka in Kanagawa Prefecture, Sasebo in Nagasaki Prefecture and in Okinawa Prefecture since 2008 and covering it up, according to the Wall Street Journal.

The wastewater treatment company has denied any wrongdoing, according to the report.

The U.S. Navy started looking into the matter in March 2018, and the Justice Department and the Federal Bureau of Investigation later joined in the investigation.

Three former employees of Kanto Kosan said the company failed to treat wastewater as contracted and falsified records and test samples, the report said.

One of the former employees said staff would typically dump partially treated wastewater from a hose inside U.S. bases when there were few other people around, and tap water was often sent in place of treated wastewater for monthly laboratory analysis to ensure a positive result, according to the report.

U.S. government data shows the navy's contracts with the company were worth more than $100 million in total since 2008, it said.

A U.S. Navy employee told the Wall Street Journal that he flagged the alleged wrongdoing to superiors about 10 times over several years. U.S. authorities are looking into whether navy staff in Japan ignored complaints about it, according to the report.

The wastewater treatment company, based in Yokosuka, said Tuesday that the report is "groundless" and that it will take legal action.

In Tokyo, Chief Cabinet Secretary Yoshihide Suga said the government is gathering information from the U.S. side on the alleged disposal of wastewater.

"We will confirm the facts first and take appropriate action," the top government spokesman said at a news conference.