The Federal Communications Commission (FCC) is launching an investigation into the nationwide CenturyLink internet outage that disrupted access to 911 lines.

FCC Chairman Ajit Pai announced on Friday that he had directed the agency’s Public Safety and Homeland Security Bureau to probe the “cause and impact of this outage.”

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

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The outage began on Thursday morning and appears to remain unresolved.

In a statement, CenturyLink said it was working "around the clock" to restore impacted services and would cooperate with any investigation.

"We take all service interruptions seriously and have had teams working around the clock to restore affected services," a CenturyLink spokeswoman said in a statement.

"We have made substantial progress in restoring and stabilizing services across the network over the last several hours. We remain focused on resolving all remaining issues. We are in contact with the FCC and policymakers and will cooperate fully with any investigation," the statement added.

The company had tweeted late Thursday night that it had identified a “network element” causing the disruption and estimated that service would be restored in 4 hours. CenturyLink later said it had identified further problems and on Friday tweeted that its "service restoration work is not complete.”

According to The Associated Press, the outage knocked out 911 service in parts of Washington state, Missouri, Idaho and Arizona.

The outage reportedly also disrupted Verizon service. CenturyLink helps with network traffic for Verizon and other wireless carriers.

Pai said on Friday that the investigation will focus on the outage’s far-reaching effects.

“This inquiry will include an examination of the effect that CenturyLink’s outage appears to have had on other providers’ 911 services,” he said. “I have also spoken with CenturyLink to underscore the urgency of restoring service immediately. We will continue to monitor this situation closely to ensure that consumers’ access to 911 is restored as quickly as possible.”

-- Updated at 3:25 p.m.