Lennar Corp.'s shares fell as much as 28% Friday after Fraud Discovery Institute Inc. alleged that the home builder operates its joint ventures "like a Ponzi scheme."

Fraud Discovery posted the allegations Friday on a Web site called lenn-ron.com. It claimed that Lennar, of Miami, has failed to disclose material transactions.

Fraud Discovery investigates what it suspects to be corporate fraud, and also provides consulting, due diligence, and expert witness testimony services. The San Diego firm was co-founded by Barry Minkow, who served more than seven years in prison, from 1988 to 1995, after being convicted of fraud while running a company called ZZZZ Best Co.

Lennar's chief executive officer, Stuart Miller, told CNBC on Friday: "To the best of my understanding, this is being authored by an angry litigant. It's my belief that the accuser, Minkow, has been hired by a litigant who lost one case and is in another, and this is just accusations in the context of litigation that is nearing resolution."

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