Other investors will face similar squeezes, said Seymour Zises, who runs Family Management Corporation, a firm that manages money for wealthy families. “I believe that the predominant selling will be among hedge funds themselves and executives who control their own companies,” Mr. Zises said.

Because margin loans are private transactions between banks and borrowers, it is difficult to know exactly how many executives or hedge funds may face margin calls as a result of the stock market’s plunge. Corporate executives must report stock sales within two days of making them. But hedge funds do not have to disclose margin calls  and in some cases the first sign that they face calls may be their abrupt collapse.

Hal Vogel of Vogel Capital Management said he was taken aback when he learned that Mr. Redstone and his privately held company, National Amusements, were so heavily leveraged.

“That suggests that there may be other cases where chief executives who are controlling shareholders or the company’s major shareholder with the same problem,” Mr. Vogel said. “These people present themselves as financially savvy and not subject to great risk associated with debt. But in fact it seems that that is not always the case.”

Knowing exactly how many firms used those strategies is all but impossible. But margin debt increased steadily from late 2002 to 2007, according to the New York Stock Exchange, which requires its member firms to report their margin loans outstanding. It peaked at $381 billion in July 2007, just before stocks peaked.

Since then, margin loans have fallen. In August, the most recent month for which data is available, they were $292 billion. But those figures do not fully account for loans made internationally, or for other strategies that funds use.