Executives with Utah-based Zions Bancorp. sought Monday to reassure investors after the bank saw a one-day stock decline worth about $1 billion in market value last week.

The bank disclosed Wednesday to the U.S. Securities and Exchange Commission that it would write off nearly $50 million of some $60 million in loans made by one of its subsidiaries, California Bank & Trust, based on apparent misrepresentations and contractual defaults by two commercial borrowers.

Zions shares dropped by more than 11% in reaction to the report. Its disclosure also combined with similar notices from two other regional banks to trigger a wider selloff in the financial sector and jitters on Wall Street that ultimately shaved nearly 300 points from the daily Dow Jones Industrial average.

In a lawsuit filed i

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