Some cellphone users in at least 14 states and Washington, D.C., were unable to call 911 for a few hours on Wednesday night, officials said.
City, county, law enforcement and emergency response officials took to social media over the course of almost five hours to warn people across the country of the disruption. AT&T said at 10:30 p.m. that the problem had had been fixed.
"Service has been restored for wireless customers affected by an issue connecting to 911," AT&T said in a statement around 10:30 p.m. "We apologize to those affected."
The telecommunications giant did not say when and how the problem began, or how many customers were affected.
The warnings began no later than 5:49 p.m., when the Monongalia County Homeland Security Emergency Management Agency in West Virginia reported the problem on Facebook. Officials continued issuing warnings until at least 10:25 p.m.
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The Hendricks County Communications Center in Indiana, a consolidated dispatch center for police, fire, and emergency medical service agencies, had said that 911 calls from AT&T customers simply failed to connect.
"We have conducted test calls locally and it will just ring," the center wrote in a Facebook post.
Ajit Pai, the chairman of the Federal Communications Commission, said on Twitter that the agency had received reports about the problem and was investigating.
Officials in at least 14 states — Alabama, Arkansas, California, Colorado, Florida, Indiana, Kentucky, Louisiana, Maryland, Pennsylvania, Tennessee, Texas, Virginia, West Virginia — and Washington had posted warnings by 9:30 p.m.
Some said the problem was sporadic while others said it was statewide or nationwide.