Wells Fargo said Thursday that customers are unable to log in to mobile and online banking and it was investigating the cause of the outage.

In a post on Twitter, the San Francisco-based bank said, "We apologize to our customers who may be experiencing an issue with our online banking and mobile app. Thanks for your patience while we research this issue. If you are impacted, please check back here for updates."

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A subsequent post said, "We're experiencing a systems issue that is causing intermittent outages, and we're working to restore services as soon as possible."

A "fire suppression" system was accidentally activated earlier Thursday at a Wells Fargo data center in Minnesota, though the fire department near the center told CNBC it responded and there was no fire. Fire Chief Tim Boehlke of Lake Johanna Fire Department told CNBC that the incident started 5 a.m. local time and Wells Fargo called the fire department at 9 a.m. it is not known whether this was related to the system interruption.

In another tweet Thursday, the bank said the system issues were "due to a power shutdown at one of our facilities, initiated after smoke was detected following routine maintenance."

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Wells is the fourth-largest U.S. bank by assets with a sprawling branch operation that reaches coast to coast. People attempting to visit its website see a message that says it's experiencing technical difficulties.

Wells Fargo's Hilary O'Byrne acknowledged the systems issue in a statement to CNBC. "We're experiencing a systems issue that is causing intermittent outages, and we're working to restore services as soon as possible. We apologize for the inconvenience."

CNBC's Ryan Ruggiero contributed reporting.

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