NEW YORK (CNNMoney) -There's a good chance your bank's website was attacked over the past week. Since Sept. 19, the websites of Bank of America, JPMorgan Chase, Wells Fargo, U.S. Bank and PNC Bank have all suffered day-long slowdowns and been sporadically unreachable for many customers. The attackers, who took aim at Bank of America first, went after their targets in sequence. Thursday's victim, PNC's website, was inaccessible at the time this article was published. Security experts say the outages stem from one of the biggest cyberattacks they've ever seen. These "denial of service" attacks -- huge amounts of traffic directed at a website to make it crash -- were the largest ever recorded by a wide margin, according to two researchers. Banks get hit by cyberattackers all the time and typically have some of the best defenses against them. This time, they were outgunned. "The volume of traffic sent to these sites is frankly unprecedented," said Dmitri Alperovitch, co-founder of CrowdStrike, a security firm that has been investigating the attacks. "It's 10 to 20 times the volume that we normally see, and twice the previous record for a denial of service attack."