The financial and banking industries are on high alert tonight as a massive cyberattack continues, with potentially millions of customers of Bank of America, PNC and Wells Fargo finding themselves blocked from banking online.
"There is an elevated level of threat," said Doug Johnson, a vice president and senior adviser of the American Bankers Association. "The threat level is now high."
"This is twice as large as any flood we have ever seen," said Dick Clarke, an ABC News consultant and former cybersecurity czar.
Sources told ABC News that the so-called denial of service attacks had been caused by hackers from the Middle East who had secretly transmitted signals commandeering thousands of computers worldwide.
Those computers -- or "zombies" -- were then used to overwhelm bank websites with a barrage of electronic traffic.