Updated February 22, 2024 at 4:36 PM ET

AT&T says it has fully restored cellphone service to tens of thousands of customers in cities across the country whose phones lost signal overnight, causing frustration and concern about disruptions to 911 dispatches.

The FBI, in a statement Thursday afternoon, said the agency was "in contact with AT&T regarding today's network outage. Should we learn of any malicious activity we will respond accordingly."

White House national security spokesperson John Kirby also referred to the outage as he addressed reporters on Thursday. Kirby said the FBI and the Department of Homeland Security (DHS) were looking into the outage. He said the Federal Communications Commission was also in touch with AT&T.

"But the bottom line is we don't have all the answers," he said.

Around 3:30 a.m. ET Thursday, outages reported by downdetector.com suddenly spiked from just a handful, peaking at more than 73,000 by around 8:20 a.m. ET.

"We sincerely apologize"

By late afternoon Eastern time, AT&T said it had "restored wireless service to all our affected customers."

"We sincerely apologize to them," the company said in an email to NPR. "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."

Kirby said the Department of Commerce faced some disruptions as a result of the outage but those were not "crippling."

Earlier, Downdetector said that Houston, Dallas, San Antonio, Chicago, Indianapolis, New York, Miami and Charlotte reported the highest number of outages. Houston had more than 2,000 reports by about 8:30 a.m. ET, while New York reported about 1,300.

However, service disruptions caused concern Thursday morning even outside those areas, with the San Francisco Fire Department announcing on X, formerly Twitter, that it was "aware of an issue impacting AT&T wireless customers from making and receiving any phone calls (including to 911)."

"We are actively engaged and monitoring this. The San Francisco 911 center is still operational. If you are an AT&T customer and cannot get through to 911, then please try calling from a landline. If that is not an option then please try to get ahold of a friend or family member who is a customer of a different carrier and ask them to call 911 on your behalf. Do not call or text 911 to simply test your phone service," the department said.

A survey conducted by the National Center for Health Statistics found in 2022 that nearly 71% of adults and 82% of children lived in wireless-only households.

Even if a cellular service provider is down, anyone with a cellphone can still place an emergency call or text, according to 911.gov. "However, calls to 911 on phones without active service do not deliver the caller's location to the 911 call center, and the call center cannot call these phones back to find out the caller's location or the nature of the emergency. If disconnected, the 911 center has no way to call back the caller," the website says.

Jared Juliano, assistant director for Prince William County Department of Public Safety Communications in Virginia, says 911 service there was never really disrupted, but calls coming in from AT&T phones did not have location information or what he describes as an "advanced caller ID."

However, he says 911 dispatchers "always verify a location even when we get locations from these numbers."

Police in the Charlotte-Mecklenburg area in North Carolina said customers were "briefly unable to contact 9-1-1. There are no disruptions to our call center's ability to receive 9-1-1 calls. Service should be returning shortly."

Other providers reported business as usual

The outage did not appear to have any real impact on other providers. Verizon said its network was "operating normally."

"Some customers experienced issues this morning when calling or texting with customers served by another carrier," a company statement said.

Likewise, T-Mobile said in a statement that its network "is operating normally."

The outages occurred around the same time as a solar flare, according to the National Oceanic and Atmospheric Administration (NOAA). The agency said the flare — "an eruption of energy from the Sun that usually lasts from minutes to hours" — reached Earth at 1:32 a.m. ET.

NOAA says such flares can affect high-frequency radio signals, but only on the sunlit side of Earth.

Copyright 2024 NPR. To see more, visit https://www.npr.org.