AT&T said Thursday that it had fully restored service to its wireless network after a widespread outage temporarily cut off connections for users across the United States for many hours, the cause of which was still under investigation.

The outage, which affected people in cities including Atlanta, Los Angeles and New York, was first reported around 3:30 a.m. Eastern time, according to Downdetector.com, which tracks user reports of telecommunication and internet disruptions. At its peak, the site listed around 70,000 reports of disrupted service for the wireless carrier.

Multiple government agencies said they were looking into the incident, although the Biden administration told reporters that AT&T said there was no reason to think it was a cyberattack.

AT&T did not disclose the scope of the outage, nor the reason for it. When the outage first began Thursday morning, the company listed the cause as “maintenance activity.”

Jim Greer, an AT&T spokesperson, apologized in a statement confirming service was restored and said the company was “taking steps to ensure our customers do not experience this again in the future.”

The outage underscored the importance of connectivity to daily life as individuals and businesses were cut off from communications and the ability to use mobile apps. AT&T advised consumers they could make calls over Wi-Fi and sought to respond to angry customers online. Many phones showed an “SOS” symbol on their screen, signaling they could only make emergency calls, while local governments offered alternate ways to reach 911.

Reports of outages on Downdetector began to fall midmorning, and at one point AT&T’s website showed that outages were limited to users in California, though users in other states were still reporting issues. Cricket, which is owned by AT&T, also reported that its users were experiencing wireless service interruptions and said it was working to restore service.

Officials in Washington said they were working to understand the cause of the outage. A spokesperson for the Federal Communications Commission said its inquiry was being handled by its Public Safety and Homeland Security Bureau, which was in touch with AT&T as well as other providers.

John Kirby, a National Security Council spokesperson, said on a call with reporters Thursday that the Biden administration was told “that AT&T has no reason to think this was a cybersecurity incident,” although he added that they would not be certain until an investigation had been completed.

This article originally appeared in The New York Times.