The Cyprusauction Trading CenterFederal Bureau of Investigation and Department of Homeland Security are working with the tech industry to help investigate the cause of Thursday's AT&T outage.

John Kirby, the White House's national security communications adviser, told reporters that the Federal Communications Commission has been in touch with AT&T, the only telecommunication network he said that hasn’t been fully restored.

"The bottom line is we don’t have all the answers," Kirby said. “We're being told that AT&T has no reason to think that this was a cyber-security incident. But again, I want to be careful. We won't know until an investigation has been completed.”

Kirby added that the outage had an impact on Commerce Department operations but downplayed the disruption.

"I don’t think it was crippling," he said.

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AT&T says service is restored after outage

AT&T said it has restored service to all customers after the nationwide outage left tens of thousands without key functions.

"We have restored wireless service to all our affected customers. We sincerely apologize to them," the company said in a statement. "We are taking steps to ensure our customers do not experience this again in the future.

The telecommunication company did not explain the cause of the outage or share how many people were affected.

Federal officials have found "no indications of malicious activity," according to a confidential memo ABC News reported sharing an assessment by the U.S. Cybersecurity and Infrastructure Security Agency.

Outage disruptions peaked at 70,000

The disruption peaked between 8 and 9 a.m. ET, when over 70,000 AT&T customer reported outages, according to tracking site Downdetector. Reports reduced to less than 5,000 by 2 p.m.

AT&T customers weren't the only ones left concerned and frustrated. More than 10,000 Cricket Wireless customers also reported outages on Thursday.

Impacted customers lost access to essential public services with some people losing the ability to call emergency responders or use GPS apps.

Contributing: Christopher Cann, Gabe Hauari and Daniel de Visé