We've seen S.E.C. investigations and lawsuits from state regulators for fraudulent marketing of auction rate securities. Banks have agreed to pay out $41 billion to their investors. We've seen two brokers indicted for misleading customers about the bonds.

Now comes a new allegation with regard to the frozen auction rate market: collusion.

Two class-action lawsuits were filed Friday in the southern district of New York—one on behalf of investors in auction rate securities and another on behalf of issuers of the bonds—against 11 banks that underwrote the securities.

The charge? Violating the Sherman Antitrust Act.

The issuer suit, brought by lawyers at Cohen Milstein Hausfeld & Toll on behalf of the City of Baltimore, accuses the banks of conspiring to falsely prop up the auction rate market by buying the securities when there were no other buyers, and similarly conspiring to simultaneously withdraw from the auctions.

Auction rate securities are long-term bonds usually backed by municipal loans or student loans that reset at auction on a short-term basis. Until February, they were marketed as low-risk, cashlike investments. When the banks withdrew from the market in February, issuers like Baltimore were forced to pay higher rates on their bonds, and investors were left holding securities they couldn't sell.

The banks named in the suit are: Citigroup, UBS, Merrill Lynch, Morgan Stanley, Lehman Brothers, Bank of America, Wachovia, Goldman Sachs, J.P. Morgan Chase, Royal Bank of Canada, and Deutsche Bank.

There's no smoking gun in the complaints, but they do effectively repurpose in a new way the damning emails that were unearthed by U.S. attorneys in New York and Massachusetts in their suits against Merrill Lynch and UBS. Those suits accused the banks of fraudulently marketing the securities even while they knew their market was freezing up.

This is the first suit that creates a time line leading up to February 13, when 87 percent of all auctions failed. The email communication indicates that officials at UBS and Merrill Lynch were well aware of what their competitors were doing. UBS's chief risk officer sends out this missive to its C.E.O.: "Watch our competitors closely; if they stop supporting auctions, we have much better freedom to stop [supporting auctions]." In another email, a UBS official relays conversations with colleagues Citigroup and Merrill Lynch on what they were doing in the auctions.

The banks clearly knew they were going to lose money if they kept supporting the auctions, but it remains a mystery why the market froze up so dramatically in mid-February. Many of the riskier auction rate securities, like those backed by subprime loans, had failed months earlier. But the banks simply stopped buying all auction rate securities very abruptly.

"There has to be an independent reason why they all pulled out at the same time," says Michael Hausfeld, partner at Cohen Milstein Hausfeld & Toll. "There has to be a connection, and we're going to make it." He hopes that if this case reaches the discovery phase, a whole new batch of emails will reveal communication among the officials at different banks.

Of course, proving an antitrust violation is no easy task. "You have to show some evidence of a conspiratorial agreement," says Columbia law professor John Coffee. "It's an uphill battle."

It's an uphill battle that's easier to fight when the Department of Justice also becomes interested in potential antitrust violations. In this case, there's no evidence of that.

Yet, anyway.