The Securities and Exchange Commission has broadened its year-long investigation of the Hunt family of Texas and now is looking into the Hunts' dealings with some of the nation's biggest banks.

Documents made public this week indicate the SEC probe has led far from its original subject -- the Hunts' massive speculation in the silver market and their purchase of a major interest in Bache Group Inc., a Wall Street brokerage firm.

The SEC now is seeking details of other Hunt transactions, including the $1.1 billion loan to the Hunts' Placid Oil Co. that was arranged last spring with the aid of Federal Reserve Board Chairman Paul Volcker. The Hunts had to borrow the billion dollars from a group of banks to pay their debts after silver prices collapsed last March.

What the SEC is looking for is still secret. The agency's mandate is to protect investors in publicly owned companies. All the banks involved in the investigation are public companies, as is Bache. SEC officials insist that the latest subpoenas are directly related to the original investigation.

The SEC has subpoenaed records of all Hunt dealings with the family's chief banker, First National Bank in Dallas, and late last month notified at least two other banks, First National Bank of Chicago and Republic National Bank of Dallas, that it wants some of their records, too.

Several other banks and brokerage houses have also been served with subpoenas in the Hunt investigation, including the nation's biggest broker, Merrill Lynch Pierce Fenner & Smith Inc. Secretary of Treasury Donald T. Regan was chairman of Merrill Lynch during the period covered by the SEC subpoenas.

Merrill Lunch is cooperating with the investigation and is not believed to be a target of the securities probe, a spokesman said yesterday. A Bache official said that firm also is furnishing documents to the SEC and does not consider itself the target of the investigation.

Thousands of pages of documents relating to the Hunts' transactions have been turned over to SEC's enforcement division in the investigation that began last April.

But now the Hunts are trying to cut off the flow of documents and keep what they describe as "confidential business information" from the eyes of the federal investigators.

Six members of the Hunt family filed a lawsuit against the SEC in Dallas on Monday, asking a federal judge to stop the SEC from obtaining records of the Hunts' dealings with The First National Bank in Dallas.

U.S. District Judge Robert Porter is expected to decide soon on the Hunts' request. Porter is a strict conservative who this week ruled that former president Carter did not have the authority to stop American firms from suing the government of Iran.

Filing of the lawsuit by the Hunts is the family's first public acknowledgement that it is the target of an SEC probe.The SEC voted to begin the private investigation a week after the price of silver ended a three-month plunge and hit bottom at $10.80 an ounce last March 27, the court papers said.

The lawsuit was filed on behalf of Nelson Bunker Hunt, W. Herbert Hunt, Lamar Hunt, Douglas H. Hunt, Houston B. Hunt and Albert Huddelston, an in-law.

Along with the lawsuit a Hunt spokesman issued a rare public statement saying "members of the Hunt family have cooperated fully with the investigation" until now, but have decided to resist "to avoid losing our rights protected by federal law."

The Hunts contend SEC lawyers have "improperly expanded the scope of their inquiry."

The SEC has "served subpoenas on banks inquiring into all banking relationships whether or not related to silver between these banks and Hunt interests," a spokesman said.

The Hunts contend the SEC has secretly broadened its investigation "without disclosing as federal law requires the nature of their inquiry."

The SEC has not responded to the Hunts' lawsuit and agency spokesmen declined to discuss the direction their probe is now taking.

The SEC investigation began with an apparently minor violation of securities law -- the failure of Nelson Bunker and W. Herbert Hunt to report their purchase of 7 percent of Bache's stock.

The Hunts' purchase of Bache stock also has raised questions about possible conflicts of interest between their roles as owners and customers of Bache. Bache was threatened with serious financial problems after silver prices collapsed and the Hunts were unable to pay their debts to the firm.

Bache has also been accused, in a lawsuit filed by a former executive, of overstating its profits by shifting unprofitable silver transactions to a subsidiary in order to make the parent company look healthier.

There is no mention of Bache in the latest subpoenas served by the SEC. The agency demanded that First International Bancshares, parent of First National Bank of Dallas turn over "all contemplated, requested, proposed or executed loan, advances, guarantees, extensions of credit or other forms of credit extended or arranged by the bank . . . either directly or indirectly . . . in amounts greater than $250,000 to or for the benefit of the Hunts."