AT&T restores service after outage

An AT&T wireless outage affected tens of thousands of customers in the US on Thursday, unable to place calls, text or access the internet as a US agency said there was no indication of a cyberattack.

AT&T hasn't said what triggered the nationwide service disruption that started early Thursday morning. By late morning, AT&T said most of its network was back online, and it confirmed Thursday afternoon that service was fully restored.

At about 5 am ET, the US Cybersecurity and Infrastructure Security Agency (CISA) reported that there was no indication of a cyberattack, according to a confidential memo obtained by ABC News. CISA is an agency within the Homeland Security Department that monitors cyber threats.

John Kirby, National Security Council spokesman, said on a call with reporters Thursday that the Biden administration was told "that AT&T has no reason to think this was a cybersecurity incident", although he added that they wouldn't be certain until an investigation had been completed.

Several government agencies including the FBI and Homeland Security were looking into the incident.

Other major providers, such as T-Mobile and Verizon, also experienced disruptions, but they were more limited than the ones from AT&T, according to Downdetector, a tracking site.

When the outage first began, AT&T listed the cause as "maintenance activity".

The most likely cause of the outage "is a cloud misconfiguration", which is "a fancy word for saying human error", Lee McKnight, an associate professor at Syracuse University's School of Information Studies, said in a statement to NBC News.

Some other cellular providers also reported outages.

When the disruption occurred, AT&T said, "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."

Throughout the day, cities urged residents to find alternative ways of reaching emergency or municipal services, like landlines or phones connected to Wi-Fi.

The San Francisco Fire Department said on social media that it was aware of an issue affecting AT&T users who were trying to call 911. "We are actively engaged and monitoring this," the fire department said, according to The New York Times. "If you are an AT&T customer and cannot get through to 911, then please try calling from a landline."

New York Police Department officials told CNN that they weren't able to make calls or use email on AT&T phones Thursday morning unless they were connected to Wi-Fi.

Massachusetts State Police warned people not to test their phone service by placing 911 calls.

"Many 911 centers in the state are getting flooded w/calls from people trying to see if 911 works from their cell phone. Please do not do this," the state police said in a social media post. "If you can successfully place a non-emergency call to another number via your cell service then your 911 service will also work."

More than 32,000 AT&T outages were reported by customers at about 4 am ET on Thursday, according to Downdetector. Outages then spiked again to more than 50,000 around 7 am and surged to more than 71,000 just before 8 am ET, with most reported in Houston, Chicago, Dallas, Los Angeles and Atlanta, according to the site.

Cricket Wireless, which is owned by AT&T and uses its network, also experienced cellular problems. More than 13,500 customers reported outages as of 8 am ET on Thursday. The number dropped to around 10,000 by 10 am.