The recent cyber attacks on a number of big U.S. banks were the work of hackers with access to sophisticated tools, Reuters reports. Security researchers describe the episodes as some of the “strongest and most complex” cyber attacks seen to date and say the hackers probably spent months sussing out the banks' defensives.

The attacks directed unusually high volumes of traffic to the banks' Websites, which slowed the sites and made it hard for bank customers to access them. Such “denial of service” attacks often involve the use of botnets – compromised computers that can be rented. Researchers say this time the botnets were made up of compromised Web servers, rather than personal computers, making them more powerful.

Banks that experienced cyber attacks include Bank of America, JPMorgan Chase, Wells Fargo, U.S. Bancorp and PNC.

See the full story here. For previous coverage, see Cyber Attacks on Banks Expose U.S. Vulnerability and Bank Websites Under Attack (Finextra).