Bank of America scrambled to restore service late Friday to its enormous customer base -- 40 million households -- which spent most of the day without access to online, mobile and telephone banking services.
As one prominent consultant called the outage "inexcusable," the bank declined to comment on the causes of the shutdown. A spokesman referred reporters to a bank tweet late Friday saying it was "still working on our technical issue."
It remained unclear whether the bank had fallen victim to another of the hacker attacks that have targeted electronic channels at big banks sporadically since September. The shutdown came as Chief Executive Brian Moynihan has been overhauling operations to better cater to customers' needs.
Mark Pepitone, a spokesman for Bank of America's technology operations, said late Friday that "the situation is improving considerably" for the online, mobile and call-in operations.
"Some customers are now able to access those channels," he said. "They're getting through."
The bank's ATMs were functioning normally, he said.
Bank of America has invested $500 million in mobile and online banking since 2008, a period in which branch transactions have dropped 35 percent. Moynihan says Internet banking is more convenient for customers as well as cheaper to operate.
Bank of America was closing in on 12 million mobile customers at the end of 2012, Moynihan boasted to a financial services conference
Given that strategy, not having backup systems in place for electronic banking and call centers "is absolutely inexcusable," said economist and bank consultant G. Michael Moebs in Lake Bluff, Ill.
"Moynihan is too good for this. He's from the trenches," Moebs said. "Somebody's head is going to roll for this one."
The outage follows cyber attacks by a shadowy hacker group in the Middle East in September that disrupted the electronic operations at the nation's largest banks: Bank of America, Wells Fargo & Co., JPMorgan Chase & Co., Citigroup Inc., U.S. Bancorp and PNC Financial Services.
Those were simple denial-of-service attacks, in which a website is deluged by automated requests for service until it breaks down. But at least one other recent case involved hackers breaching bank security systems and making off with customers' funds.
Federal prosecutors last month charged a Russian, a Latvian and a Romanian with creating a computer virus that infected more than 40,000 U.S. computers in an effort to steal customers' bank-account data and other information.
The "Gozi" virus led to the theft of unspecified millions of dollars, said U.S. Attorney Preet Bharara in Manhattan.
Customers who tried to sign on to Bank of America online and mobile services on Friday were greeted with a text message advising them to go to ATMs or branches.
"Our site is temporarily unavailable," it said. "We know your banking is important and appreciate your patience."
Calls to Bank of America's telephone banking service went unanswered as well.