Attorney Bruce Barket told Newsday that Tankleff will seek about $35 million.
Government attorney Christine Malafi says Suffolk County will vigorously defend itself.
Tankleff's parents were found on Sept. 7, 1988 in their home in Belle Terre, a well-to-do waterfront enclave. Arlene Tankleff was bludgeoned. Seymour Tankleff was stabbed and died about a month later.
Tankleff's attorneys plan to file the federal suit Tuesday in Central Islip.
NEW YORK AND TRI-STATE AREA NEWS