AT&T outage caused by software update, company says

A temporary network disruption that affected AT&T customers in the U.S. Thursday was caused by a software update, the company said.

AT&T told ABC News in a statement ABC News that the outage was not a cyberattack but caused by "the application and execution of an incorrect process used as we were expanding our network."

"We are continuing our assessment of today’s outage to ensure we keep delivering the service that our customers deserve," the statement continued.

The software update went wrong, according to preliminary information from two sources familiar with the situation.

Sources have told ABC News that there was nothing nefarious or malicious about the incident.

The outage was not caused by an external actor, according to a source familiar with the situation. AT&T performs updates regularly, according to the source.

In an earlier statement to ABC News, AT&T said some customers were "experiencing wireless service interruptions" and advised them to make calls over Wi-Fi.

The company issued an update later Thursday afternoon saying that its network had been fully restored.

"We have restored wireless service to all our affected customers. We sincerely apologize to them. Keeping our customers connected remains our top priority, and we are taking steps to ensure our customers do not experience this again in the future," the company said in a message on its website.

Two sources briefed on the situation told ABC News that the FBI and Department of Homeland Security (DHS), among other agencies, had been urgently investigating to determine whether the AT&T outage was the result of a cyberattack or a hack, or simply some sort of technical malfunction.

As of 5 a.m. ET, the U.S. Cybersecurity and Infrastructure Security Agency (CISA) reported, according to a confidential memo obtained by ABC News, that "the cause of the outage is unknown and there are no indications of malicious activity." CISA is an agency within DHS tasked with monitoring cyber threats.

The FCC has been in touch with AT&T to figure out what caused the outage, according to National Security Communications Advisor John Kirby.

Kirby told reporters Thursday afternoon that DHS and the FBI were looking into the outage as well and working with the tech industry and network providers to see what can be done "from a federal perspective to enhance their investigative efforts to figure out what happened here."

"The bottom line is we don't have all the answers," he said. "We're working very hard to see if we can get to the ground truth of exactly what happened."

Several police departments and municipalities warned local residents of what they described as a nationwide outage. In turn, officials urged callers to contact emergency services by alternative means.

"There is a nationwide AT&T outage that is preventing wireless customers from making and receiving any phone calls (including to 9-1-1)," the Charlotte-Mecklenburg Police Department, which serves the Charlotte, North Carolina area, said in a post on X.

The county government in Fairfax, Virginia, released a similar warning.

"There is a nationwide AT&T outage that is preventing wireless customers from making and receiving any phone calls (including to 9-1-1)," the Fairfax County Government said on X. "Try calling from a landline or ask a friend or family member to call 9-1-1 on your behalf."

In response to an earlier request from ABC News, CISA said they had no comment on the outage.

AT&T serves more than 100 million customers across mobile and broadband services, according to the company's website.

Verizon and T-Mobile both told ABC News that their respective networks are not experiencing outages but customers may experience difficulty when contacting individuals affected by outages at other providers.

"Verizon's network is operating normally. Some customers experienced issues this morning when calling or texting with customers served by another carrier. We are continuing to monitor the situation," a Verizon spokesperson said.

T-Mobile similarly told ABC News, "We did not experience an outage. Our network is operating normally. Down Detector is likely reflecting challenges our customers were having attempting to connect to users on other networks."

This is a developing story. Please check back for updates.