James FitzGerald and Sam Cabral
BBC News, London and Washington
Tens of thousands of cell phone users across the US have reported mobile service outages on Thursday morning.
Downdetector.com, which tracks outages, showed reports indicating no service or no signal after 04:00 EST (09:00 GMT).
AT&T said some of its customers were experiencing wireless service interruptions but that three-quarters of the network had now been restored.
Other carriers including Verizon and T-Mobile told the BBC that their networks were operating as normal.
But both carriers claimed customers were likely logging issues after making unsuccessful attempts to contact users of another provider.
AT&T customer complaints peaked at just over 74,000, with hotspots in the south and east, Downdetector shows.
“Our network teams took immediate action and so far three-quarters of our network has been restored,” the company said in a statement.
“We are working as quickly as possible to restore service to remaining customers.”
The company did not give details about the possible cause of the outages.
Cricket Wireless, a small carrier, also confirmed service interruptions, writing in a statement: “We are working urgently to restore service to them.”
Neither company has yet explained the cause of the outage but both encouraged the use of Wi-Fi calling until service was restored.
Customers with UScellular and Consumer Cellular, two other small carriers, were also reporting issues.
Users have said their phones are displaying the SOS message, leaving them unable to make calls or access services.
The complaints comprise issues with calls, texts and internet access, with most reports indicating no service or no signal.
Downdetector’s heatmap shows major population centres including Los Angeles, Chicago, Houston and Atlanta are among those facing the highest number of outages.
Some have also reported experiencing difficulties with 911 service, with officials across the country advising the use of a landline, social media or a cell phone with a different carrier in the event of an emergency.
The US government is concerned about the widespread outage, with the FBI and Department of Homeland Security both investigating what happened, US National Security Council spokesperson John Kirby told a press briefing.
The US Cybersecurity and Infrastructure Security Agency is “working closely with AT&T to understand the cause of the outage and its impacts, and stand ready to offer any assistance needed”, said Eric Goldstein, its executive assistant director for cybersecurity, in a statement.
Citing a confidential memo from earlier Thursday, ABC News reported that CISA officials believe “there are no indications of malicious activity”.
