Briefly

The NASD is expanding a probe of the world’s biggest brokerage firms for possible improper sales of hedge funds to individual investors, three people with direct knowledge of the matter said.

The industry regulator sent letters to firms including Merrill Lynch & Co., Citigroup Inc. and UBS seeking details on customers who bought “retail hedge fund products” for $50,000 or less, said the people, who declined to be identified because the probe isn’t yet public.

The NASD, which was formerly known as the National Assn. of Securities Dealers, declined to comment. Merrill, Citigroup and UBS also declined to comment.