The Federal Reserve asked Capital One Financial Corp. to respond to questions that appear aimed at determining whether the proposed acquisition of ING Groep NV's U.S. online-banking business would create a bank so large and complex that its failure would pose a risk to the financial system.

Capital One, the ninth-largest bank in the U.S. by deposits, was pressed by the Fed in an Aug. 29 letter for details about "the nature and dollar volume" of financial activities in which both companies are involved. The two-page letter also listed various financial markets that were troubled during the financial crisis, such as commercial paper, mortgage-backed securities, lending to foreign institutions through foreign-exchange swaps and derivatives trading.

Fed officials asked Capital One to describe any markets where the McLean, Va., bank and ING Direct USA are market makers and to report information about exposure to counterparties.

Several banking-industry experts said they couldn't recall any previous proposed acquisitions where the Fed asked some of the questions sent to Capital One.

Following last year's Dodd-Frank financial-overhaul law, the Fed now must consider whether specific mergers and acquisitions would increase overall risk to the financial system.