Police are questioning a number of people allegedly involved in the sale of a substance believed to be uranium, on suspicion they may have broken the law by handling the strictly controlled radioactive material, investigative sources said Thursday.

Tokyo police have identified the seller and multiple men who successfully bid for the material online, and have taken them in for questioning on a voluntary basis. Authorities have also asked the Japan Atomic Energy Agency to confirm whether, as suspected, the material is depleted uranium or uranium concentrate, the source said.

The powder was enclosed in a glass tube and put on sale under the name of “Uranium 99.9%” on Yahoo’s auction website. The Nuclear Regulation Authority notified police of the sale in January last year after discovering the advertisement in November 2017.

An initial test of the powder proved that it is radioactive, according to the sources.

A Japanese nuclear regulation law bans people other than approved businesses and organizations from transferring nuclear fuel materials such as depleted uranium.

Anyone found to have violated the law can be subject to imprisonment for up to one year or a fine of up to ¥1 million.

As uranium found in nature only consists of about 0.7 percent of the uranium 235 isotope, which easily splits, it needs to be concentrated in order to be used for nuclear power generation or nuclear weapons. Waste produced during the concentration process is called depleted uranium.