Citadel Securities and Robinhood Markets Inc. won dismissal of a proposed class-action lawsuit brought by retail investors who accused the firms of colluding during January's meme-stock frenzy.

U.S. District Judge Cecilia Altonaga in Miami said that the plaintiffs failed to show there was any agreement between Citadel Securities and Robinhood to act in concert. In a Wednesday ruling, she dismissed the case without prejudice, giving the investor group until Dec. 20 to possibly file an amended complaint.

The lawsuit alleged that Citadel Securities amassed a substantial short position in GameStop Corp. and other stocks that exploded in value, and that the market-maker pressured Robinhood to stop customers from purchasing those shares, which the online brokerage did on Jan. 28.

"The request from a market maker to limit order flow sent to it is not equivalent to a demand to restrict trading," Altonaga wrote in her decision.

"We are pleased that the court agreed that there is no basis for the plaintiffs' conspiracy theories and summarily dismissed the case," Citadel Securities spokesman David Millar said in an emailed statement.

Robinhood spokeswoman Jacqueline Ortiz Ramsay likewise said in an emailed statement, "This further confirms that the conspiracy theory of collusion has no basis in fact."

The legal battle has drawn attention to the complex machinery behind the execution of trades. Robinhood sends customer orders to Citadel Securities and other trading firms to be carried out, and accepts remuneration from those firms in what's called payment for order flow.

Citadel Securities said it first learned of Robinhood's trading restrictions on certain stocks during January's meme-stock frenzy from Twitter, rebutting accusations that the two firms colluded. E*Trade Securities and Interactive Brokers were also named as defendants in the case.

The judge said the investor group were only inferring that such pressure was applied based on "a few vague and ambiguous emails between two firms in an otherwise lawful, ongoing business relationship."

She also said the fact that some firms not named in the suit imposed trading restrictions of their own in the wild trading environment rendered the claim that the named defendants engaged in a conspiracy "implausible."

The plaintiffs said that the firms identified in their lawsuit could collude because of the close-knit and secretive nature of their industry. The judge said these were "generic descriptions" that did not alone prove a conspiracy took place.

Though the investor group still has a chance to respond to the judge's decision to dismiss the case, Robinhood and other defendants "will continue to prevail" given the lack of evidence, Bloomberg Intelligence litigation analyst Elliot Stein wrote in a note Thursday.

"Brokers' parallel conduct, even if intentional, was justified in light of clearinghouse collateral requirements," Stein said.

The case is In re: January 2021 Short Squeeze Trading Litigation, 21-2989, U.S. District Court, Southern District of Florida (Miami)

Katherine Doherty and Annie Massa report for Bloomberg News.

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