The Federal Communications Commission is investigating what caused a 911 outage that affected AT&T wireless customers in multiple states on Wednesday. The Dallas-based company has revealed little about the outage, which has since been fixed.

Here’s what we know:

WHAT HAPPENED?

Based on reports from law enforcement and government agencies, AT&T customers were unable to call 911 from their cellphones for a brief period Wednesday night. In many cases, police and sheriff’s departments provided an alternate phone number for residents to use in case of an emergency.

The company declined to answer questions today about the outage and instead issued a brief statement saying service had been restored to the affected customers.

In Arkansas, one sheriff’s office shared a message it received from AT&T that blamed “a service outage that is impacting the ability to deliver AT&T Mobility wireless 911 calls in your area.” The message noted there could be issues with 911 calls or certain “location information.”

HOW MANY PEOPLE WERE AFFECTED?

It’s unclear, but likely thousands. Maybe millions.

Officials in Baltimore, Houston, Washington, D.C., and other cities reported the outage, along with authorities in more than a dozen states.

Several law enforcement agencies described the outage as “nationwide,” and FCC Chairman Ajit Pai said on Twitter that his agency received reports of a widespread outage. An FCC spokesman said the agency was trying to determine how many people were affected but noted the investigation had just begun.

AT&T has 147 million wireless customers in the U.S. and Mexico, according to its website .

IS THIS UNPRECEDENTED?

No. Large-scale 911 outages have happened in the past.

Wireless carrier T-Mobile had two separate outages on one day in August 2014 that together lasted three hours and affected all 50 million of its customers at the time, according to the FCC. The company reached a $17.5 million settlement with the FCC in 2015.

An outage in April 2015 lasted up to six hours and affected more than 11 million people in seven states, the FCC said in a report. Several companies, including CenturyLink and Verizon, were fined as a result.

WHAT’S NEXT?

Pai, the FCC chairman, said today that an investigation was underway.

“Every call to 911 must go through,” Pai said in a statement. “So when I first learned of yesterday’s outage, I immediately directed FCC staff to contact AT&T about it and the company’s efforts to restore access to emergency services to the American public.”

Lisa Fowlkes, acting chief of the FCC’s Public Safety and Homeland Security Bureau, said the commission’s public safety employees were already on the case.

“We will fully investigate this outage and determine the root cause and its impact,” she said.

Associated Press writer Tali Arbel in New York contributed to this report.