A federal judge has further whittled down a lawsuit that claims Nassau District Attorney Kathleen Rice conspired to prosecute an investor and his attorney at the behest of campaign contributors, but will allow key portions of the suit to proceed.

Smithtown attorney Robert Del Col and his client, Ted Doukas of Florida, sued Rice and officials of DataTreasury, a Texas company founded in Melville. They claimed that campaign contributions made to Rice by DataTreasury officials led her office in 2009 to charge Del Col and Doukas with extortion.

Nassau prosecutors said the pair had tried to extort DataTreasury officials in exchange for Doukas keeping quiet about what he said was his role in creating the lucrative technology the company licenses to banks. Nassau County Judge Meryl Berkowitz dismissed the charges in 2010, and state appellate courts have refused to allow Rice's office to reinstate them.

Doukas and Del Col sued Rice and company officials in federal court. District Judge Margo Brodie dismissed several claims in July, and in a decision this week she threw out claims of false arrest and imprisonment.

Allowed to proceed are conspiracy claims against all defendants and malicious prosecution claims against the DataTreasury defendants. All the parties claimed the decision helped their cases.

"I think it's a dramatic step in the right direction," Del Col said. The conspiracy claim is the core of the suit and will allow him to prove what he calls a pay-to-prosecute scheme, he said. "It leaves me exactly where I want to be."

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Both DataTreasury and Nassau prosecutors said they acted properly in pursuing extortion charges, and noted that those charges were dismissed because of what Berkowitz ruled was an improper grand jury presentation and not on the merits of the case.

Scott Mollen, a Manhattan attorney for several company officials, said he was confident all the claims will fail in the end.

"We are pleased that the courts have now dismissed almost all of this frivolous lawsuit, and we believe the rest of it will be dismissed in due course," said John Byrne, a spokesman for Rice.

Del Col and DataTreasury continue to battle in three separate lawsuits in Suffolk County, one of which is in trial.