Hackers target online banking at PNC, Bank of America, JPMorgan Chase, Wells Fargo and U.S. Bank

If you had trouble accessing your bank accounts online this week, you weren't alone.

Hackers targeted five major U.S. banks this week, blocking online access for millions of customers.

PNC bank was hit Thursday.

Bank of America, JPMorgan Chase, Wells Fargo and U.S. Bank were targeted earlier in the week.

All of the sites appeared to be functioning normally Friday morning.

Hackers can cause banking websites to fail through "Denial of Service" attacks. They gain control of thousands of computers worldwide, then direct so much traffic to the banks' websites that they crash, experts say.

A group calling themselves "Izz ad-din Al qassam" has claimed responsibility for the attacks. They said they would attack the financial industry in retaliation to the controversial anti-Islam film that sparked protests around the world.

Security leaders in the U.S. believe hackers in Iran may be part of this group.

In the latest attack, hackers have not been able to steal money from bank accounts, but experts fear that could be the next step now that this major security flaw has been exposed.