JPMorgan Chase's (JPM) website has stumbled again roughly three weeks after a cyberattack.

The nation's biggest bank by assets took to Twitter on Monday afternoon to tell customers the website was "experiencing intermittent issues" and to recommend customers use its mobile service while the company worked "to get things up to full speed." As of late Monday, the site had been affected for three hours.

A source close to JPMorgan Chase said the company is investigating the cause of the outage.

On March 12, JPMorgan Chase confirmed that its website had endured a denial of service attack, a type of electronic assault in which a website receives a fusillade of messages that can render it inoperable.

Monday's outage is not believed to have resulted from a denial of service attack, according to the source.

On Monday afternoon, the website Sitedown.co, which tracks website failures, logged 194 reports of people being unable to log in to their accounts at JPMorgan Chase within the preceding 24 hours, compared with 200 reports in the past seven days.

The interruption at JPMorgan Chase follows a renewed wave of denial of service attacks on the nation's financial institutions. On Thursday, American Express was hit by a cyberattack that left some customers unable to access their accounts. Both Wells Fargo, the nation's fourth-largest bank by assets, and TD Bank suffered similar attacks in the past two weeks as well.

The al-Qassam Cyber Fighters, a hacktivist group, has claimed responsibility for a series of denial of service attacks since September that have impeded websites at some of the nation's biggest banks. The group has vowed to continue its campaign until YouTube takes down a trailer for an anti-Muslim film. The group said in an email on March 26 it had hit Wells Fargo, JPMorgan Chase, BB&T (BBT), PNC Financial (PNC), Citigroup (NYSE:C), U.S. Bancorp (USB), SunTrust (STI) Fifth Third Bancorp (FITB) and other companies in the latest round of attacks, although the source of the attacks on Amex and TD Bank remains unclear.