Bank of America's website was sluggish and intermittently unavailable for many users on Tuesday, in an outage that the bank hadn't explained by the end of the day.

Bank of America (BAC) spokesman Mark Pipitone said the company is "working to ensure full availability," and that online banking is available "although some customers may experience occasional slowness."

The bank's official Twitter account sent similar messages to customers who complained about the issues.

The majority of Bank of America's customers were able to access the site, which remained online throughout the day, Pipitone said. Still, many customers were unable to connect because of the site's slowness.

Data from Keynote, a testing and monitoring company, showed that the problems began around 10 a.m. ET and were still ongoing as of 6:10 p.m.

"It's possible that something Bank of America has done on its end, some kind of change, caused the problem," said Aaron Rudger, Keynote's senior marketing manager of Web performance. The bank's homepage looked different before the trouble began, he said.

As of about 6 p.m. Tuesday, Rudger said, a "small percentage" of website requests were being fulfilled successfully, but the majority were still met with errors.

On Wednesday morning, Rudger said that the bank appeared to have "rolled out some change that improved the situation and modestly reduced the long time-out failures" on Tuesday night. By mid-morning on Wednesday, Keynote's data showed the site's performance had stabilized.

If the problem is indeed due to system updates that Bank of America rolled out, it wouldn't be the first time. The bank experienced nearly a full week of outages in October 2011, and later explained that the problems stemmed from a combination of technical difficulties and heavier than normal traffic. The bank had been migrating its online banking to a new platform and deploying new tools for customers when the problems began.

On blogs and Twitter, some hacker groups were claiming responsibility for the outage. But there was no evidence to support those claims, and several recent ones turned out to be hoaxes.

Earlier this month, a person affiliated with the hacktivist collective Anonymous said the group took down the web hosting service GoDaddy, and in June the group UGNazi claimed responsibility for downing Twitter. Both outages were later revealed to be technical issues.

"I can assure you we continuously take proactive measures to secure our systems," Bank of America's spokesman said in response to a question about whether the company had seen any signs on Tuesday of a cyberattack.