(Jared Soares for The Washington Post)

Federal authorities said they will investigate a nationwide outage that kept AT&T customers from being able to contact 911 on Wednesday night.

The Federal Communications Commission said its investigation would determine how many people were affected by the outage as well as what caused the issue, which was reported by law enforcement agencies and first responders across the country Wednesday and confirmed by AT&T, a Dallas-based telecommunications giant.

“Every call to 911 must go through,” Ajit Pai, the FCC chairman, said in a statement announcing the probe. “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.”

Pai said that he had directed FCC staffers “to track down the root cause of this outage.”

AT&T has not said yet what it thinks caused the issue, nor has the company said how it wound up restoring 911 service. In a pair of messages posted on Twitter, AT&T warned of an “issue affecting some calls to 911 for wireless customers” and then, 41 minutes later, said the issue was resolved.

A spokeswoman for AT&T declined Thursday to respond to a list of questions regarding the breadth of the outage and how many attempted calls did not go through. She provided a statement from the company saying they would give more details to the FCC for its probe.

“We take our 911 obligations to our customers very seriously and will be sharing additional information with the FCC,” the company said in a statement.

The FCC does not yet know how many customers were affected, how many calls were attempted and how many states, in total, were impacted, a spokesman said Thursday afternoon.

The National Emergency Number Association, a nonprofit group focused on 911 issues, said reports indicated that some callers trying that number received “fast busy signals” Wednesday.

Law enforcement agencies from Washington to Colorado and from Massachusetts to Texas reported 911 problems Wednesday. In the District, a spokesman for the city’s Office of Unified Communications said that D.C. officials began responding to the outage about 9 p.m., about 90 minutes before AT&T said the issue was restored.

However, it was not immediately clear how widespread the outage was. In Colorado, for example, police in Denver and Lakewood, a nearby city, warned of problems with 911, but Aurora, another city in the area, reported that its own tests did not find the problem there.

Similarly, the outages were not uniform across other areas. In the Washington area, officials in the District, Prince George’s County and Alexandria all reported problems, while Montgomery County police said preliminary testing did not find any trouble contacting its 911 center.

During the outage, agencies directed people needing emergency help to call or text nonemergency numbers, which meant that the locations of callers were not always immediately known.

Issues involving 911 outages have drawn public attention in recent years. In 2014, the FCC released a report delving into a widespread 911 failure that year affecting more than 11 million people across seven states. In Washington state alone, every resident appeared to have lacked “fully functioning 911 service” for up to six hours, the FCC said.

The outage, which the FCC said “was caused by a software coding error” at a Colorado call-routing facility, ultimately kept more than 6,600 calls from reaching call centers during that six-hour window. It stretched from Florida and the Carolinas to California.

“What is most troubling is that this is not an isolated incident or an act of nature. So-called ‘sunny day’ outages are on the rise,” the report stated. “That’s because, as 911 has evolved into a system that is more technologically advanced, the interaction of new and old systems is introducing fragility into the communications system that is more important in times of dire need.”

After a June 2012 storm caused outages at a number of call centers — affecting millions in Northern Virginia for up to four days — the FCC investigated and determined that some problems were avoidable.

The Washington Post found that the 911 emergency network across the Washington region had suffered a series of outages over a period of two years, despite such networks being designed to continue operating even when power lines and other infrastructure elements are knocked out.

Lisa Fowlkes, acting chief of the FCC’s Public Safety and Homeland Security Bureau, which deals with issues including 911 calls and emergency communications, vowed that the agency would figure out the scale of what happened in the outage Wednesday night.

“The FCC’s public safety professionals are on the case,” Fowlkes said in a statement. “Access to 911 emergency services is essential for all Americans, especially the most vulnerable. We will fully investigate this outage and determine the root cause and its impact.”

This story has been updated to include a response from the FCC about not knowing the extent of the impact yet. 

Further reading:

From 2012: Verizon’s 911 failures began long before the derecho hit