SAN FRANCISCO - Wells Fargo & Co. said Monday that it would file with the Justice Department and the Federal Trade Commission its intention to purchase more than 15%, or $15 million, of voting securities of First Interstate Bancorp.

Wells Fargo, based in San Francisco, will make the so-called Hart-Scott- Rodino Act filings in order to show its "commitment" to its unsolicited offer to buy First Interstate for more than $10 billion in stock, said Wells Fargo spokeswoman Lorna Doubet.

Ms. Doubet would not say if the filings signal that Wells is prepared to follow up its letter to First Interstate's board with an actual tender offer to shareholders. But observers said the Hart-Scott-Rodino Act filings appear to a step in that direction.

"This is probably a precursor to a tender offer," said Campbell K. Chaney, a stock analyst in San Francisco with Rodman & Renshaw Inc.

A First Interstate spokesman refused to comment on the matter.

Mr. Chaney said the possibility of an extended takeover battle appears to be pushing down the stock of both companies. At midday, First Interstate's stock was trading at $129.75, down $7.50, while Wells Fargo was trading at $216.125, down 62.5 cents.

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