PNC weathered a slowdown in its online and mobile services Friday that left some customers unable to access their accounts.

The Pittsburgh-based company in the morning took to social media to tell customers it was investigating the problem and working to restore access for all customers. As of Friday afternoon, the bank continued to look for the cause of the disruption. "We do not have more information at this time," PNC said in a message posted on Facebook.

PNC spokeswoman Marcy Zwiebel said that some customers continued to be able to log in to their accounts while others experienced difficulty.

Though PNC stressed that the cause of the outage remained to be determined, the interruption follows a roughly three-week letup in cyberattacks acknowledged by the nation's biggest banks. On Tuesday, the Al Qassam Cyber Fighters, a group that has taken responsibility for most of the distributed denial of service attacks that have been launched against banks since September, announced a new string of targets. The list did not include PNC; it did include Regions, M&T Bank, Unionbank, Principal Financial, Ameriprise, State Street, Citizens Financial, and Wells Fargo.

Wells Fargo on April 4 endured the second assault on its websites the company had experienced in as many weeks. The incident followed a string of denial of service attacks on such institutions as American Express (AXP), BB&T (BBT), TD Bank and JPMorgan Chase (JPM).

In all, more than 13 financial institutions, including PNC, have experienced cyberattacks since September.

The slowdown also comes amid efforts to strengthen the nation's cyber defense. The House of Representative recently passed a bill that would encourage sharing of information about cyber threats among private-sector firms and the U.S. Government.

The Senate is expected to consider similar legislation, while the White House is leading a parallel effort to develop a system for threat sharing.