Nightly News | April 04, 2013
>>> back now as promised with our nbc news investigation tonight into a sustained series of attacks into this nation's online banking system. it started last fall. it is estimated month after month. the most recent attack came today against wells fargo . the questions, of course, who's responsible, what does it mean for the safety of americans' bank accounts ? we get some answers tonight from tom costello.
>> reporter: the attacks have been relentless. week after week the nation's biggest banks' online sites -- among them citibank, chase, wells fargo , bank of america -- paralyzed by hackers. customers who tried to log in are often unable to connect. nbc news technology correspondent bob sullivan .
>> this is a major assault on the u.s. financial industry . it's been going on for seven months. no one has seen anything like it before.
>> reporter: in the last six weeks the 15 largest u.s. banks have been off line for a total of 249 hours.
>> as recently as monday we saw a big attack.
>> reporter: outside of san francisco , keynote monitoring added up the waves of cyber attacks for nbc news.
>> the attacks not only appear to be increasing in frequency but also in the complexity of how -- or the sophistication of how the attacks are occurring.
>> reporter: who is behind it? a group calling its al cassam cyber fighters claims retaliation for the youtube video that mocks islam. security experts believe the attacks are too sophisticated and evolving for a single group to pull off.
>> the speculation is that it's the iranians. there is definitely evidence that it would take a nation state to launch this kind of attack.
>> reporter: intelligence sources believe iran could be responsible, noting the attacks began soon after it was hit with tighter international banking sanctions for its nuclear program. iran though denies it's responsible. so far america's banks so no account holder has lost money. they insist customers don't need to take extra precautions to protect their accounts.
>> the attackers are very determined. we're determined, too. we're determined to stop these attacks and work in unison to stop these attacks.
>> reporter: america's banks appear caught in the cross hair hs of a new cold war fought in cyberspace. tom costello, nbc news, washington.