Chuck Burton/AP Photo

For a third consecutive business day Bank of America’s homepage is suffering serious disruption. The nation’s largest bank by deposits, with 12 percent of Americans’ bank accounts and over a trillion dollars in deposits, told ABC News “the cause is still under review, but we have ruled out hacking, malware, or any link to the debit fee. We have not said the issues were due to volume either.”

Despite repeated requests from ABC News, the bank has refused to explain the source of these now chronic service problems.

Cyber-security experts counter the only explanation for outages on four of five consecutive days is a so-called denial of service attack.

“The only reasonable conclusion is that they are under attack,” says Steve Gibson, an Internet security expert.  ”A site of that size should be expected to handle huge volume with no trouble at all…the only time we ever see anything like this is when some major site has upset a group of hackers.”

Denial of service attacks occur when a site is inundated with an enormous number of apparently legitimate requests. It explains why the site’s homepage alone seems to be suffering the brunt of the disruptions.

“What’s scary,” says Gibson, “is that it only takes the whim of a teenager to take a site like this off line,” explaining that denial of service attacks are relatively simple to engineer.

The outages, which includes the bank’s hobbled home page as well as delays and difficulty in accessing online banking, began Friday morning, a day after the bank announced it would charge a $5 monthly fee for account holders using their debit cards.

Bank of America says it would waive that fee for those with “Premium” accounts – which require a minimum monthly balance of $5000.

ABCNews’ Bert Rudman contributed to this report