Many AT&T customers were left without cellphone service Thursday thanks to a widespread outage that began in the early morning and Slabu Exchangelasted until roughly 3 p.m. ET, when the company confirmed it had restored service to affected customers.

Following the incident, AT&T and federal organizations are investigating the cause of the disruption, assessing the possibility of a cyberattack. AT&T has since completed an initial review, though it says it will continue to look into the issue to prevent future occurrences.

Here's what we know so far about the cause of the AT&T outage.

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What caused the AT&T outage?

An initial review by the company found the disruption was caused by the "application and execution of an incorrect process used as we were expanding our network," or, more simply put, a technical error.

AT&T said it was continuing its assessment of the matter as of Thursday night.

Was the AT&T outage a cyberattack?

According to a statement posted by AT&T Thursday evening, the network outage was not the result of a cyberattack.

The Federal Bureau of Investigation (FBI) and Department of Homeland Security (DHS) partnered with AT&T to launch an investigation into the outages into the possibility of an attack.

The White House's national security communications adviser John Kirby said Thursday afternoon, “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.”

When was the AT&T outage fixed?

AT&T customers first began experiencing service disruptions on Thursday in the early morning hours. Between 8 and 9 a.m. ET, more than 70,000 AT&T customers were reporting outages, according to Downdetector, a tracking site that relies on user submissions.

The company did not officially confirm the number of customers affected.

AT&T put out a statement at 11:15 a.m. ET, saying it had restored service to three-fourths of impacted customers. Another statement released at 3:10 p.m. confirmed that service was back online for all affected customers.