Bank officials and analysts are puzzled over why shares of a Chicago bank holding company were trading at more than 1,600% of its normal volume on Tuesday.

About 94,600 shares of River Forest Bancorp's stock changed hands Tuesday in one of the heaviest days of trading for the institution.

That's about 1,627.3% of its normal daily trading volume of a few thousand shares.

And at least 65,000 shares were exchanged on Wednesday.

Tuesday's unusual volume also comes on the heels of an even stronger surge last Thursday, when 160,000 shares were traded on the stock market.

Officials and analysts were at a loss to explain the heavy trading, especially in the absence of any rumors that often drive stock surges.

"I don't know who the buyers were or who the sellers were," said Robert J. Glickman, president and chief executive of $1.7 billion-asset River Forest.

The surge may have been caused by one or two large institutional investors suddenly deciding to sell shares for some unknown reason, said Daniel L. Westrope, director of research for Howe Barnes Investments Inc. in Chicago.

Mr. Westrope said the institution is very attractive to investors because it maintains a high returns on equity and assets, averaging 20% and 1.5%, respectively, over several years. The bank is also a active player in the student loan market and has kept its operational costs low, he added.

And since the bank is normally only lightly traded, "it's never been a problem finding buyers for this stock," which at $32.25 is trading at about 160% of book value, Mr. Westrope said.

"We think it's a good company, and we continue to recommend their stock as being undervalued currently in the market-place," he said. But "we don't know any specific reasons why so much would trade all of a sudden."

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