Police are trying to confirm whether the product is radioactive, he said, with local media reporting investigators are questioning "a number of people" suspected of involvement in the sale and purchase of the substance.

An initial test of the confiscated powder found it to be radioactive, Kyodo News said.

The Nuclear Regulation Authority received a report about a year ago about what was being billed as "uranium" on a popular auction site operated by Yahoo! Japan.

"We were asked, 'Is it OK? How is it possible?' We immediately called Yahoo!, which then took down the product," an atomic agency official told AFP.

The case was referred to police as the law requires special permits for transferring the ownership of uranium and other radioactive materials, he said.

Tokyo Metropolitan Police declined to comment on the matter.

Officials at Yahoo! Japan were not immediately available for comment.

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, Kyodo News said.

Authorities have also asked the Japan Atomic Energy Agency to confirm whether the material is depleted uranium or uranium concentrate, Kyodo said.

The product was enclosed in a glass tube and put on sale under the name of "Uranium 99.9%" at a Yahoo online auction website, Kyodo said.

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