The cause of one of the largest ATM outages in U.S. history became the topic of widespread speculation Tuesday, as cyber experts scrambled to figure out why most of Wells Fargo & Co.'s 12,000 ATMs went dark for several hours.
The San Francisco-based bank stayed largely mum on the nationwide outage, which was unusually long in both its duration and scope. Thousands of Wells Fargo customers were unable to get their cash until the automated teller machines were fixed late Monday evening.
It's unclear exactly what happened. The bank would say only that the outage was caused by "an internal systems issue" and was not related to an external cyber attack or a security breach.
However, Wells Fargo's silence did not keep many in the bank technology world from speculating on the possible causes. One popular theory is that the ATM network crashed as Wells Fargo was trying to upgrade its system to protect against a cyber attack. The bank regularly updates the encryption codes in its ATMs to prevent hackers from stealing the personal identification numbers that people enter into the machines, say people who work closely with the bank.
An internal e-mail from Wells Fargo, obtained by the Star Tribune, said the outage affected 9,500 ATMs nationwide and occurred at 2:54 p.m. Central time Monday.
"The irony here is that, in their zeal to protect consumers and their network, Wells Fargo may actually have taken its system down," said Richard Crone, a bank technology consultant from San Carlos, Calif.
Wells Fargo came under some criticism Tuesday for its handling of the incident. In Minnesota, many customers who tried to withdraw their cash on Monday afternoon were greeted with "Out of Service" messages, and branch tellers with no information. Angry customers made their opinions known via Twitter, barraging Wells Fargo's Twitter account (@Ask_WellsFargo) with questions and complaints.
To many customers, the bank tweeted, "Hi, ATMs are now operational. We apologize for any inconvenience."