Those were the reports this past weekend, but TechCrunch chief Michael Arrington last night dismissed them as nonsense. Not because JPM doesn’t want a big piece of Twitter, but because it already has a big piece of Twitter.

A source very close to the situation tells me that Arrington is on the money.

Here’s how it apparently worked: Chris Sacca, an early Twitter investor and advisor, recently raised around $1 billion for a Twitter-specific secondary fund. Much of that fund’s commitments came from JPM – most likely via its $1.2 billion Digital Growth Fund (which didn’t hold a first close until February 8). Sacca then bought up around $400 million worth of shares, from Twitter co-founder Ev Williams, certain company employees and existing shareholders Spark Capital and Union Square Ventures.

The purchases were done at various valuations, the highest of which was $4.5 billion.

If Sacca/Twitter rings a bell, it may be because we wrote last fall about how he was raising dedicated funds to buy shares in the micro-messaging service (albeit much smaller dollars than we’re talking about today).

Two additional notes:

(1) It would seem that the JPM Digital Growth Fund is, at least in part, a secondary fund-of-funds. Actually makes sense, given the background of its team. Or, perhaps, lack of team. JPM Asset Management doesn’t have any ex-tech operators on staff (at least none I could find), and the fund’s regulatory filings suggest that management has been outsourced to a Cayman Islands-based fund management group called Walkers Global.

(2) This deal shouldn’t raise any 500-shareholder concerns, at least insofar as the JPM fund’s involvement. Even if JPM Digital Growth has 1,000 limited partners, the vehicle would appear to be a blind capital pool (as opposed to a broker, which Goldman Sachs seemed to be with its much-publicized Facebook fund). As such, the SEC most likely would view JPM as a single investor. Kind of like how Union Square and Spark are single investors, even though they each have dozens of underlying LPs. That said, it wouldn’t be too surprising if Sacca and/or JPM soon receive a letter of inquiry from the SEC, as part of its larger investigation into secondary trades… Or maybe they already have.