Some of the largest and best-known banks in the country were recently targeted by hacking attacks that crippled their websites, and experts say these could be the biggest attacks ever.

In the days since Sept. 19, the websites for Bank of America, JPMorgan Chase, Wells Fargo, U.S. Bank and PNC Bank have all been hit by what are known as denial of service attacks that brought the online destinations to a standstill, according to a report from CNNMoney. Security experts say that these attacks would have taken large amounts of coordination and resources, and are therefore the largest of their kind ever.

"The volume of traffic sent to these sites is frankly unprecedented," Dmitri Alperovitch, co-founder of the security firm CrowdStrike, told the news site. "It's 10 to 20 times the volume that we normally see, and twice the previous record for a denial of service attack."

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It's believed that the hackers responsible for these attacks would have had to use thousands of servers and directly targeted each bank individually, the report said. Each bank was targeted in sequence, beginning with Bank of America and Chase on the first day. These attacks were so effective, in fact, that PNC says that even a day later, they were still affecting some users' ability to access to its site.

Further, experts say that the hackers likely had to set up massive "botnet" servers that essentially used malware to hijack other servers and make them all work in concert to bring down the targeted sites, the report said. That may be an indicator that groups like Anonymous or LulzSec, two of the larger "hacktivist" groups on the Web, were not behind the attacks, as use of botnets on that scale is not part of their usual modus operandi.

However, customers of the affected banks can rest assured that these attacks did not result in the exposure of any of their personal or financial data, the report said. Further, ATM systems and other unrelated networks were likewise not affected.

Whenever a consumer is worried that their bank or credit card account might have been hacked, they should carefully check over their transaction history to see if they can spot any unfamiliar transactions made, as these are the best indicators that such an incident resulted in identity theft.

Image: ericnvntr, via Flickr