VIENNA — With an aggressive style that stood out in the staid world of Austrian banking even more than her bouffant red wig, Sonja Kohn made few friends gathering billions for Bernard L. Madoff from wealthy investors in Russia and across Europe.

Now, she has even fewer. Mrs. Kohn has dropped out of sight, leaving the firm she founded, Bank Medici, in the hands of Austrian regulators, who took it over last week.

Embarrassment from investing heavily with Mr. Madoff could explain wanting to disappear from public view. But another theory widely repeated by those who know Mrs. Kohn is that she may be afraid of some particularly displeased investors: Russian oligarchs whose money made up a chunk of the $2.1 billion that Bank Medici invested with Mr. Madoff.

“With Russian oligarchs as clients,” said a Viennese banker who knew Mrs. Kohn and her husband socially, “she might have reason to be afraid.”