A group led by Greenberg said in a regulatory filing that it had hired an investment bank, Perella Weinberg Partners, to advise it on the stake.
"Greenberg is trying to make up his mind whether to let go of his AIG holdings or not, and he's seeking Perella's advice," said Donald Light, an analyst with Celent LLC. "My guess is he'll probably let them go."
The filing stated Greenberg and his companies were seeking advice on "the business, operations, financial condition and underlying value" of AIG.
The news sent AIG shares plunging to a 12-month low. The shares were down $2.88, or 5 percent, to $51.39 in afternoon trade on the New York Stock Exchange after falling as low as $50.57 earlier.
Through a spokesman, Greenberg declined to comment, as did AIG and Perella Weinberg. Greenberg's stake in AIG is worth about $17 billion.
Greenberg has been fighting with AIG since February 2005, when he was ousted as CEO amid an accounting scandal. There have been frequent lawsuits between the two sides.
In November, Greenberg said in a regulatory filing that he was seeking "strategic alternatives" for AIG, a phrase often used to indicate a takeover or proxy fight.
But in December, the New York State Department of Insurance told Greenberg he had violated state insurance laws because he had not sought its permission to take control of the company, according to a subsequent filing.