Bloomberg reports that Federal Trade Commission officials have been interviewing small businesses that sell through Amazon about the company's business practices.

The interviews lasted roughly 90 minutes, and interviewees were asked how big a portion of their revenue Amazon sales made up, three merchants told Bloomberg.

Experts told Bloomberg the length of the interviews and the number of workers assigned to them indicated the FTC was in the early stages of an official investigation into Amazon.

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A new report from Bloomberg appears to indicate that the Federal Trade Commission is 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 came from Amazon sales in comparison with other e-commerce sites such as Walmart and eBay, and the interviews were said to last roughly 90 minutes.

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

Read more: Big tech warned by top regulator: We will break you up if we have to

Experts told Bloomberg, however, that its findings suggested the FTC could indeed be in the early stages of an official investigation, 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," a former FTC official named Michael Kades said. "They're spending lots of time with witnesses and trying to really understand what they're saying." The Bloomberg Intelligence analyst Jennifer Rie said the questioning had the hallmarks of the "background phase" of an investigation.

Amazon declined to comment when contacted by Bloomberg, and was not immedately available for comment when contacted by Business Insider. The FTC declined to 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 an investigation into Google. Amazon is also already the subject of an antitrust investigation by the European Union, officially announced in July.