Amazon.com Inc.'s Japan unit is "fully cooperating" with a police investigation into possible sales of child pornography, the company said in an emailed statement.

Aichi Prefectural Police conducted raids on the Internet retailer's Tokyo headquarters and a distribution center in Chiba Prefecture on Jan. 23, seeking evidence that Amazon Japan's website may have been used by sellers to trade porn goods, including photo books of underaged girls, Tokyo Shimbun reported, citing an unnamed official involved in the probe.

Amazon's usage rules "prohibit the sale of any illegal products through the company's website," Amazon Japan said in the statement.

Photobooks and DVDs of scantily clad pre-pubescent girls — part of an "idol" industry that generated $615 million from bands, models and starlets in 2013 — were available in October at Japan's three largest retail websites — Amazon Japan K.K., Rakuten Inc. and Yahoo Japan Corp.

When Japan last year revised its laws to ban possession of child pornography, the wording left the so-called junior idol industry as a gray area. The probe of Amazon Japan may indicate police are now starting to crack down on mainstream retailers that have continued to carry the products.