Hackers breached the firewall of several U.S. banks this week, including JPMorgan Chase ($53.66 0.31%), Reuters news reported.

Apparently, the mega bank's site was hacked Tuesday, following cyber attacks on Bank of America ($13.24 0.03%), Citigroup ($50.52 -0.01%) and Wells Fargo ($40.24 0.23%).

JPMorgan restored the site Tuesday, but advised investors that future attacks are possible with cyber warriors growing more bold in their attacks on financial firms.

Cyber attacks on companies, particularly on U.S. banks, are getting worse, Army General Keith Alexander, head of the U.S. military's Cyber Command, said in a separate hearing of the Senate Armed Services Committee.