(CNN) Special counsel Robert Mueller has subpoenaed the Trump Organization for business documents -- some of which are related to Russia, according to The New York Times -- a move that represents the latest evidence that the probe seems to have widened to include far more than simply Russia's attempts to meddle in the 2016 presidential election.

The details of the subpoena are not available. That fact makes it hard to know exactly what Mueller is looking for, what it has to do with Russia or the Trump Organization's business in the country and why he needed to subpoena the documents as opposed to simply asking for them.

But what we do know is that President Donald Trump -- and his lawyers -- have warned Mueller against a widening of the probe to include things that happened before Trump became a candidate or having to do with his real estate business.

In an interview with The New York Times in July 2017 , Trump was asked whether Mueller looking into his personal finances -- not involving Russia -- would amount to a "red line"-like violation in his mind. Here's how he responded:

"I would say yeah, I would say yes. By the way, I would say, I don't -- I don't -- I mean, it's possible there's a condo or something, so, you know, I sell a lot of condo units, and somebody from Russia buys a condo, who knows? I don't make money from Russia."

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