(CNN) Emergency cell phone service in locations across the country has been disrupted by an outage at Louisiana-based communications company CenturyLink.

The company tweeted Friday morning it had "discovered some additional technical problems" as it worked on restoring service. The interruption began Thursday.

"We continue to make good progress with our recovery efforts and we are working tirelessly until restoration is complete," CenturyLink said.

We discovered some additional technical problems as our service restoration efforts were underway. We continue to make good progress with our recovery efforts and we are working tirelessly until restoration is complete. We apologize for the disruption. — CenturyLink (@CenturyLink) December 28, 2018

Federal Communications Commission Chairman Ajit Pai called the outage "completely unacceptable" in a statement. He said he spoke to CenturyLink about his concerns and has ordered an investigation.

"When an emergency strikes, it's critical that Americans are able to use 911 to reach those who can help," Pai said.

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