The Paul Manafort trial began this week. (Bill Hennessy/CBS/Reuters)

The news earlier this week wasn't greeted with much fanfare: Special counsel Robert S. Mueller III had referred the cases of potential foreign lobbying violations by Tony Podesta, Vin Weber and Gregory Craig to the Southern District of New York. Despite these cases arising out of his Russia investigation — which makes them fair game for Mueller — he decided not to deal with them personally. And that made sense, given that they don't appear to have much to do with Russian interference in the 2016 election.

But that also may be the very important point — especially with regard to the just-begun Paul Manafort trial.

Critics of Mueller's investigation, including President Trump, have seized upon the Manafort trial as an example of what Mueller hasn't produced — specifically, anything proving collusion between the Trump campaign and Russia. Manafort is charged with financial and other crimes that predate the 2016 election.

“These old charges have nothing to do with Collusion,” Trump said Wednesday in a tweet, again calling the investigation “a Hoax!”

But experts say the fact that Mueller is farming out things like the Podesta, Weber and Craig cases — and even the Michael Cohen case, which also went to SDNY — reinforces the idea that Manafort is still seen as a potentially key figure in the Russia collusion case. And that goes for basically anything else Mueller has apparently kept under his purview, up to and including the Seychelles meeting, Erik Prince, George Nader and Roger Stone. Mueller has now shown he'll hand off multiple things that don't further that specific aim, and everything that hasn't been handed off would logically still seem potentially pertinent to the collusion probe.

“My best guess is that the most obvious answer is the correct one: He took on Manafort himself, because Manafort is obviously a key witness to many of the events most central to his investigation and wanted to retain the capacity to plea-bargain with him,” said David A. Super, a law professor at Georgetown University. “If he does not think Craig, Weber and Podesta know anything central to his work, it makes sense to refer them out, both to conserve his limited staff resources and to avoid concerns that his inquiry is spreading more than is necessary.”

But — and this is a key point — that doesn't exactly explain why Mueller is prosecuting Manafort but farmed out the Cohen case. Cohen, like Manafort, is widely viewed as someone who could flip on Trump and who is under investigation for things apparently unrelated to the campaign. So why would Mueller refer the Cohen case (along with Podesta-Weber-Craig) to SDNY but not the Manafort one?

The one obvious difference would seem to be that Manafort was a key figure in the Trump campaign and had existing ties to Russian interests that could conceivably be involved in the case's future. While Cohen may know derogatory things about Trump — and reportedly is even telling people Trump knew about the Trump Tower meeting — Manafort may still be viewed as more of a potential collusion witness.

“They’re the same in that, ultimately, Muller wants to use both to flip on the president," said former federal prosecutor David F. Axelrod. "The difference, it seems to me, is that Manafort was at the Trump Tower meeting and was part of the Trump campaign. … Cohen is more incidental.”

Super noted that we may only know a portion of the evidence Mueller has on Manafort -- and that Mueller could be keeping evidence related to other alleged crimes secret for now.

“Mueller may be seeing a more coherent and comprehensive picture,” Super said. “If so, it would be quite natural for him to try to prove it a piece at a time. That is what prosecutors often do.”

Added Patrick J. Cotter, a former federal prosecutor who is now with the Greensfelder law firm: “Compare Manafort, who if a [foreign lobbying]/tax/fraud prosecution or two can persuade him to cooperate, could definitely shed light on those central questions.”

The referral in the Cohen case could perhaps be understood as Mueller wanting New York to handle a case in which the alleged crimes took place in New York. But the Podesta, Weber and Craig cases don't really seem to have much to do with New York. Instead, it seems Mueller is keeping his focus narrowly on Russian interference and not broadening it out into the wide-ranging “witch hunt” of which he's been accused.

That means we can assume, more than we could a few days ago, that the things Mueller has kept under his purview are in service of that more narrow focus — rather than him simply trying to prosecute every potential crime that arises from his investigation.