Federal regulators are looking into the source of an outage that left millions of AT&T wireless customers in 14 states and the District of Columbia without access to 911 emergency services for several hours on Wednesday.

The New York Times reports that some AT&T customers were unable to call 911 for about five hours on Wednesday evening.

Shortly after the issue began around 5:30 p.m. ET emergency service providers in the affected areas began warning residents about the disruption.

The Times reports that the Monongalia County Homeland Security Emergency Management Agency was the first organization to report the issue.

A short time later AT&T addressed the outage with a Tweet informing customers that an unspecified issue was affecting some 911 calls.

Federal Communications Commission chairman Ajit Pai Tweeted that the agency was receiving reports of the “widespread” outage.

AT&T informed customers on Twitter who were sharing their frustration over the outage that it was working on the issue and hoped to have it resolved soon.

Emergency service providers in Alabama, Arkansas, California, Colorado, Florida, Indiana, Kentucky, Louisiana, Maryland, Pennsylvania, Tennessee, Texas, Virginia, West Virginia, and Washington, D.C. continued to post warnings about the outage, noting that it was at times sporadic or widespread, until it was resolved about five hours later.

Around 10:30 p.m ET, AT&T announced on Twitter that the issue had been resolved. However, the company did not offer an explanation for why the issue occurred in the first place.

FCC Chairman Pai followed up the AT&T announcement that service had been restored by noting that the agency would be looking into the “root cause” of the outage.

In the past, the FCC has fined service providers for 911 outages. In July 2015, T-Mobile agreed to pay $17.5 million after a nationwide outage. Before that, in April 2015, the FCC fined CenturyLink $16 million for its actions during a massive 911 outage.

In March 2015, Verizon was required to pay $3.4 million for failing to notify officials of a large scale 911 service outage in April 2014.

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