A brewing FTC antitrust investigation into Amazon is looking more and more serious

A new report from Bloomberg appears to indicate that the Federal Trade Commission ramping up an investigation into Amazon over its marketplace.

Attorneys and at least one economist have been conducting lengthy, in-depth interviews with small businesses that sell their products through Amazon, three of the merchants interviewed told Bloomberg.

They were specifically asked how much of their revenue comes from Amazon sales as opposed to other e-commerce sites including Walmart and eBay, and the interviews lasted roughly 90 minutes.

Vox reported in June that the FTC had started speaking to the company's competitors to gain some insight into its business practices. It did not name who these competitors were, and noted that these talks did not necessarily mean a full-blown investigation was underway.

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Experts told Bloomberg, however, that its report suggests the FTC could indeed be in the early stages of an official probe, given the duration of the interviews and the resources devoted to the operation.

"Early in an investigation, that's a sign of staff doing a serious job," said ex-FTC official Michael Kades. "They're spending lots of time with witnesses and trying to really understand what they're saying." Bloomberg Intelligence Analyst Jennifer Rie said the questioning had the hallmarks of the "background phase" of an investigation.

Both Amazon and the FTC declined to comment when contacted by Bloomberg, and neither were immediately available for comment when contacted by Business Insider.

Amazon is far from the only tech company to be put under an antitrust microscope.

The FTC opened an investigation into Facebook in June, and earlier this week 50 state attorneys general announced a probe into Google. Amazon is also already the subject of an antitrust investigation by the EU, officially announced in July.