Cyber attacks against large banks are showing no signs of slowing down.

On Thursday, BB&T and Wells Fargo reported site problems, saying they believed them to have stemmed from cyber attacks.

For Wells Fargo, the assault came just a week after the bank reported that its website was running slowly as a result of abnormally high traffic, which the bank suspected was part of a cyber attack.

BB&T said Thursday that its site underwent an attack in the early afternoon. Just two days before, the Winston-Salem, N.C.-based bank had reported another such attack.

In what’s becoming a growing trend among companies, the banks turned to Facebook to inform customers of the problem and, the banks hoped, to assuage their fears.

On Tuesday, BB&T posted to its Facebook page that it underwent a “targeted cyber event designed to prevent you from accessing online banking and other Internet services.” BB&T said it was “working to restore service as soon as possible.”

Wells Fargo, on Facebook, told customers that Thursday’s incident was a “denial-of-service,” or “distributed-denial-of-service,” attack. In such strikes, a computer network is flooded with Web traffic, slowing down or disabling access to websites. In its Facebook post, Wells likened it to a “cyber traffic jam.”

In online posts, Izz ad-Din al-Qassam Cyber Fighters, a so-called hacktivist group, has admitted to launching cyber attacks against major U.S. banks. In one posting, from Jan. 29, the group named Wells Fargo, Bank of America, BB&T and JPMorgan Chase as among the banks it has targeted.

According to an October report from the U.S. attorney general, cyber crime is a concern.

“The department has advanced the fight against global cyber threats, which can disrupt critical infrastructure such as the power grid, nuclear power plants, financial and banking institutions, transportation systems and vital communication systems,” the report says.

Brian Davis, spokesman for BB&T, the 15th-largest U.S. bank by assets, said the bank has been hit by cyber attacks since September.

“In a general sense, over the last couple of weeks we have been affected by these … events, just like several other financial institutions in the same time frame,” he said.

During an attack, most visitors to BB&T’s website will experience intermittent outages, and “some people might (see) slowness. Some people might see an error message,” he said,

On Thursday morning, a Charlotte Observer reporter could not access BB&T’s site. Instead, the following message appeared:

“At this time, we are experiencing intermittent outages on BBT.com. This may prevent access to the site or use of our online products and services.”

Davis, though, said the bank’s technical staff reported that the site was working normally Thursday morning.

Wells Fargo, on Facebook, suggested that its customers keep trying to log on to its website “as the disruption is usually intermittent.”

Wells is the fourth-largest U.S. bank by assets.

Davis, the BB&T spokesman, said BB&T’s customers might encounter problems on its website while the bank is using “mitigation efforts” to address the cyber attacks. But it’s rare that those efforts affect the use of BB&T’s site, he said.