Tremendous gains in third-quarter net income at some of the largest banking companies did not keep analysts from registering concerns Monday about possible bad news looming in the future.

Though Citigroup Inc. boosted earnings more than threefold, to $2.4 billion, and Bank of America Corp. outpaced that with a 475% rise, to $2.2 billion, those who track banking industry performance said they doubted such results can be sustained for long.

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