A temporary network disruption that affected AT&T customers in the U.S. Thursday was caused by a software update gone 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.
AT&T declined to comment on the situation to ABC News Thursday evening.
The disruption prompted federal agencies to investigate whether the outage was caused by a cyberattack.
In an earlier statement to ABC News, AT&T confirmed the outage and advised customers to make calls over Wi-Fi.
“Some of our customers are experiencing wireless service interruptions this morning. We are working urgently to restore service to them. We encourage the use of Wi-Fi calling until service is restored,” an AT&T spokesperson said.
Later Thursday afternoon, AT&T issued an update 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.
A man walks past the AT&T store in New York’s Times Square, June 17, 2015.
Brendan McDermid/Reuters, FILE
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 outages were 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 outages, according to National Security Communications Advisor John Kirby.
Kirby told reporters Thursday afternoon that DHS and the FBI are looking into the outages 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 outages.
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.