The Federal Communications Commission is investigating an ongoing nationwide telecommunications service outage at CenturyLink Inc. that has impacted access to 911 emergency phone services across the U.S.

CenturyLink initially confirmed the outage in a Dec. 27 tweet and continued to tweet about its work to fully restore services as of midday ET on Dec. 28, at which time the company tweeted that service restoration efforts were still not complete, despite "good progress." CenturyLink also indicated that it had found "some additional technical problems" in troubleshooting the Dec. 27-28 outage.

"When an emergency strikes, it's critical that Americans are able to use 911 to reach those who can help," FCC Chairman Ajit Pai said in a Dec. 28 statement. "The CenturyLink service outage is therefore completely unacceptable, and its breadth and duration are particularly troubling."

Pai said the FCC investigation "will include an examination of the effect that CenturyLink's outage appears to have had on other providers' 911 services."

The FCC's public safety and homeland security bureau will lead the investigation.

CenturyLink is the second largest U.S. communications provider to global enterprise customers.