The plunge in American insurance stocks widened to include companies such as Aetna Inc. and Humana Inc. on concern New York Attorney General Eliot Spitzer’s probe of the industry will drag down profits.

Shares of Marsh & McLennan Cos., the insurance broker sued by Mr. Spitzer last week, extended their decline, losing about half their value in the past four trading sessions.

Mr. Spitzer accused Marsh of rigging bids and steering clients’ business to property and casualty insurers that paid it the most fees.

MetLife Inc., the second-largest American life insurer, and UnumProvident Corp., the biggest American disability insurer, said they received new subpoenas from Mr. Spitzer, raising the possibility that the investigation is extending to sales of employee benefits such as life, disability, and health insurance.

“It’s hazardous to touch these stocks right now,” said Thomas Wille, who oversees the equivalent of $200 million in American stocks at Verwaltungs-und Privat-Bank AG in Zurich. “Basically the motto is: ‘If Spitzer is around, sell.’ Who needs the aggravation?”

Mr. Spitzer said in an interview he was looking at many types of insurance. He declined to name which ones.

“Many will be implicated by the time we are done,” Mr. Spitzer said.

Shares of UnitedHealth Group Inc., the biggest American health insurer, fell $6.85, or 9.3%, to $66.50 in New York Stock Exchange composite trading; Humana dropped $1.18, or 6.15 percent, to $18.02. Aetna declined $11.57, or 12%, to $86.17, while Cigna Corp. fell $6.85, or 10%, to $59.73.

Aon Corp., the second-biggest insurance brokerage, declined 9.7% to $19.20 after the Wall Street Journal reported Mr. Spitzer was targeting the Chicago-based company. Aon steered clients’ business to insurers that let Aon arrange their reinsurance con tracts, the Journal said.

“There have been allegations raised about issues pertaining to the reinsurance market,” Mr. Spitzer said in yesterday’s interview. He declined to say whether his office planned to take action against Aon.

The share plunge at Marsh, which wiped out $11.4 billion in market value since October 14, intensified investor calls for its chief executive officer, Jeffrey Greenberg, to step down.

Mr. Spitzer said he would not negotiate a settlement with current management.

“You’ve got to rework the culture of Marsh,” said James Huguet, who manages $1.4 billion at Great Companies LLC and has been selling Marsh shares. “This happened on Jeff’s watch.”

New York-based Marsh canceled a conference call with investors Monday and had its debt rating cut by Moody’s Investors Service. Its shares dropped yesterday $1.54, or 6%, to $24.03.

“To sit there and say nothing isn’t the right strategy,” Mr. Huguet said. “They’ve got to communicate with the shareholders to tell them what they are going to do to get this thing straighten out.”

UnumProvident, based in Chattanooga, Tenn., said it won’t make new fee agreements with brokers until it completes an internal review of its procedures. The company’s shares fell $1.33, or 9.8%, to $12.19.

MetLife shares slipped 52 cents, or 2.4%, to $33.67 after saying it uncovered no evidence of bid-rigging with brokers. The New York-based life insurer said it paid brokers $25 million in contingent fees last year. Such fees are the subject of Mr. Spitzer’s suit.

The Morgan Stanley Health Care Payer Index, which includes 12 health insurers, fell 9.4%, the biggest decline in five years.

“Every company in the sector is under pressure today,” Cigna spokesman Wendell Potter said. “We are cooperating with the attorney general’s office and responding to requests for information.”