A Forbes reporter was ordered to testify about a six-year-old interview with Donald Trump, and that gives a glimpse into the fraud investigation of the former president's family business.

The grand jury heard from the publication's chief content officer Randall Lane, who had fought the subpoena on First Amendment grounds but was ordered by a judge to confirm some elements of his reporting about Trump's net worth, and NBC News investigative reporter Tom Winter explained the significance.

"The judge narrowed the scope of that testimony," Winter told MSNBC's "Morning Joe." "The judge appears to have narrowed the scope to essentially saying, are the things you published the truth? What we're reading here in this document, is this what you published? Typically we don't even get that far. Normally in courts when you publish something it is self-authenticating."

Winter said it's highly unusual to drag a reporter into court to confirm published reports, which can be read into the record as evidence on their own, but he said prosecutors may be looking to determine whether Trump knowingly inflated his net worth to gain a financial advantage while also deflating his property valuations for tax purposes.

READ MORE: Trump under investigation for golf club tax scheme that forced 'honest taxpayers' to pay him refunds

"I think the big question is were there representations made to Forbes with any individual, whether it be Trump or a member of the family or member of the Trump Organization, saying, 'See, these are the documents we showed to this bank or this investor and did they get those representations?'" Winter said. "If so, did that match up with what was presented to those banks, those investors, and if those documents were false, then I think that gets them closer to intent or at least who was fronting these documents. I think that might be a little bit more relevant to the investigation."

"But, look, it's a tricky slope because, at the end of the day, you really do need to prove that the crime existed," he added. "Again, no crime to make misrepresentation to Forbes or anybody else but the question is, did they make misrepresentations to those various banks or investment houses? I think that's still the key."

Here's why a six-year-old interview could doom Trump in fraud case www.youtube.com