Capital One has proposed a $9 billion acquisition of online bank ING Direct. (Peter Dejong/AP)

the decision. Officials from the agency did not return phone calls requesting further comment.

“This is a thoughtful and deliberate process and we appreciate the thoroughness of the Fed’s review,” Capital One spokesperson Tatiana Stead said in a statement. “We look forward to their final decision and the positive impact the acquisition will have on our customers, associates, shareholders and our communities.”

The acquisition has come under fire from consumer groups, who say the merger will create another behemoth bank whose failure could cripple the financial system.

Consumer advocates made more than 100 phone calls over the past six days, urging the agency to strike down the merger, according to the National Community Reinvestment Coalition, a District-based nonprofit that has led the opposition movement.

“This is a small sign that perhaps the era of rubberstamping bank mergers is over. This should be a welcome sign for taxpayers, considering that it will be their money at risk should Capital One grow to be ‘Too-Big-to-Fail,’ ” said National Community Reinvestment Coalition chief executive John Taylor, in a statement.

Capital One agreed in June to pay $6.2 billion in cash and $2.8 billion in stock to Dutch financial services firm ING Groep NV for the online bank.

ING Direct comes with 7 million customers, $80 billion in deposits and $92 billion in assets, $41 billion of which are mortgage loans. Those added would catapult McLean-based Capital One from being the eighth-largest U.S bank measured by deposits to the fifth, with an estimated $206 billion in deposits.

Banking experts expected the Fed to fast track the acquisition because McLean-based Capital One had largely steered clear of mortgage-backed securities investments, derivatives trading and haphazard loan underwriting that kneecapped other firms.

Under Dodd-Frank, the Fed must consider the risk of a deal to the stability of the financial system. The agency has yet to define what constitutes a systemically risky merger, making its decision on Capital One all the more important as it may set the tone for future deals.

Capital One does stand to expand its dominance of the revolving-credit market, if it uses the injection of deposits to back more credit cards and other consumer loans. Rolling up ING Direct, however, would give Capital One total deposits that would still be a distant fifth from the next largest bank, Wells Fargo, with $853.6 billion in deposits.