Bank of America customers had problems accessing their accounts for six days. After the site appeared back to normal Wednesday, the bank attributed the troubles largely to a system upgrade.
David Owen, who heads online and mobile banking at Bank of America, said the slowness and time-outs customers experienced were the result of a "multi-year project" to upgrade its online banking platform.
He said that the testing of certain features and high traffic at the end of the month also contributed to the delays.
"That convergence created slowness across the platforms," Owen said.
When the problems first surfaced Friday morning, Owen said the bank cast a "wide net" and worked with law enforcement officials to quickly rule out the possibility of third-party interference. In the meantime, the bank said publicly Friday and afterward that it does not break out causes for website problems.
Bank of America Corp., based in Charlotte, North Carolina, is the largest U.S. bank by deposits and has 29 million online customers.
The website delays meant some customers who normally bank online have had to go to branches or ATMs to access their accounts in recent days.
Owen said the company was about 60 percent through the upgrade and didn't rule out the possibility of site problems in the future.
"As far as when we declare this over, we're taking it day by day," he said. "But right now it's business as usual."
The upgrade is intended to improve the look and navigation of the site, Owen said. He said all customers can see at least part of the improvements.
Bank of America customers also had difficulty accessing their accounts in January and March. The company said in both cases that the problems resulted from routine system upgrades.
The latest outage was significant for its duration.
Shawn White, vice president of Keynote Systems Inc., which monitors the performance of company websites, noted that banks have invested heavily in promoting the convenience and safety of their online services. He said the length of Bank of America's outage could seriously damage its customers' confidence in the company.
When outages occur, companies need at least to keep customers updated, said Jacob Jegher, an online banking analyst with research firm Celent. That's especially true for banks because of the sensitive nature of the information they handle, he said.
Even if the bank doesn't give an explicit reason for the outage, Jegher said that companies' participation in social media has led customers to expect a greater level of communication.
"We're in an environment now where electronic banking is a mainstream channel," he said. "Any extended outage is unacceptable _ particularly for a bank with this many customers."
The problems began a day after the company said it would start charging a $5 monthly debit card fee. That announcement Thursday was met with customer outrage; an online petition at Change.org asking the bank to reconsider the decision had more than 132,500 signatures by Wednesday afternoon.
Several major banks have introduced new or higher fees for checking account customers in the past year. The industry says the changes are needed because of a new regulation that limits the fees they can collect from merchants whenever customers swipe their debit cards.