The key detail in the two securities fraud complaints filed so far against JPMorgan Chase isn't that CEO Jamie Dimon told analysts that news reports about the bank's risky credit default swap position were a "tempest in a teapot." Even though that's the statement both complaints pinpoint as best evidence so far of the bank's alleged deception, to understand the shape this litigation is likely to take, you have to check out the class period both complaints (here and here) assert. It's unusually short for a securities class action, beginning on April 13 -- when Dimon made the fateful "tempest" comment -- and ending on May 10, the day the bank disclosed losses of $2 billion in CDS trades, with more to come.

Under the rules the U.S. Supreme Court established in its 1975 opinion in Blue Chip Stamps v. Manor Drug Stores, only investors who bought or sold JPMorgan shares within that window can be part of the class suing for fraud. Shareholders who bought the bank's stock before Dimon's comment on April 13 are not in the suit as it's currently framed, even if they subsequently lost millions after the May 10 disclosure. So before JPMorgan shareholders can file a suit and make a bid to be named lead plaintiff in the class action, they have to be sure they bought in that one-month window. (The law permits claims by sellers as well, but as a practical matter, it's much harder for them to show that they suffered losses tied to the alleged fraud.)

That's one reason the first complaints against JPMorgan weren't by the large institutional investors that typically end up as lead plaintiffs. Pension and healthcare funds have to figure out when they purchased the bank's shares and what their losses in the class period were. They buy and sell all the time, but one veteran shareholder class-action lawyer told me the short window will limit the universe of institutional investors with large losses. He also said that many institutional investors have made significant gains in JPMorgan investments in the last couple of years and may be reluctant to sue the bank.

My colleague Tom Hals at Reuters has written about some of the hurdles JPMorgan shareholders will have to leap over in a securities class action based on the CDS trading losses, but three plaintiffs' lawyers I talked to all said their pension fund clients are looking at the case. "There are major institutions that made significant purchases in direct or indirect reliance on the comments Dimon made on April 13," said Darren Robbins of Robbins Geller Rudman & Dowd, who filed the first JPMorgan securities-fraud class action Monday, on behalf of an individual investor. "I would suspect that, yes, there will be significant interest (from them)."

"No one's rushing," said another shareholder lawyer. "People want to see more as it comes out." (They'll get more to chew over after Dimon testifies before the Senate Banking Committee, as early as next month.) Under the securities litigation laws, shareholders have 60 days from when the first complaint is filed to bring their own claim and make a pitch to be lead plaintiff.

The bank hasn't disclosed what firm will defend it in the securities class actions, and a JPMorgan spokesperson didn't return my call requesting comment. Among the likely candidates, all of which have represented JPMorgan in securities litigation, are Paul, Weiss, Rifkind, Wharton & Garrison (recently called in to take over Bank of America's defense in the Merrill merger class action); Sullivan & Cromwell; Simpson, Thacher & Bartlett; and Wachtell, Lipton, Rosen & Katz.

(Reporting by Alison Frankel)

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