FCC officials will conduct an investigation in an effort to determine the “root cause” of a widespread outage that prevented AT&T wireless customers from completing 911 emergency calls last night, according to FCC Chairman Ajit Pai.

Pai initially announced the investigation last night via his Twitter account and reiterated the plans in a prepared statement released today.

“Every call to 911 must go through,” Pai said the 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. 

“I also spoke with Randall Stephenson, AT&T’s chief executive officer, and stressed the urgent need to restore service and to communicate with first responders, as well as AT&T customers, about the status of operations.  Additionally, I announced last night that I have directed Commission staff to track down the root cause of this outage.”

According to multiple media reports, AT&T wireless customers in several states received a busy signal or heard continuous ringing with no answer when dialing 911 during the outage. These AT&T customers did not experience any disruption in normal service when dialing 7-digit or 10-digit phone numbers.

With this in mind, many 911 jurisdictions throughout the country used social-media outlets to advise AT&T customers to call their administrative lines—or text the public-safety answering point (PSAP), if they were capable of receiving them—as an alternative to dialing 911 during the outage period.

AT&T acknowledged the problem at 8:49 p.m. last night on its Twitter account, stating, “Aware of issue affecting some calls to 911 for wireless customers. Working to resolve ASAP. We apologize to those affected.” Less than an hour later, AT&T said it had “resolved” the 911-outage problem and repeated its apology.

Although AT&T announced the resolution to the outage, the company has not provided any explanation why the outage occurred, as of the publication time for this article.

However, the FCC expects its investigation will uncover the cause of 911 outage.

“The FCC’s public safety professionals are on the case,” Lisa Fowlkes, acting chief of the FCC’s public safety and homeland-security bureau, said in a prepared 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.”

AT&T today released a statement in response to the FCC’s investigation announcement.

“We take our 911 obligations to our customers very seriously and will be sharing additional information with the FCC,” according to the AT&T statement.