The AT&T network service outage that left tens of thousands of customers across the U.S. without calling or data capabilities for hours is now under investigation by New York’s top prosecutor.

New York Attorney General Letitia James announced Thursday she will be investigating the causes the Feb. 22 outage and AT&T’s response to the disruption.

The blackout saw users unable to make calls, send texts, access internet for up to 12 hours and left many unable to call 911, James’ office said in a news release.

“Americans rely on cell service providers for consistent and reliable service to help them with nearly every aspect of their daily lives,” James said. “Nationwide outages are not just an inconvenience, they can be dangerous, and it’s critical that we protect consumers when an outage occurs.”

She encouraged impacted New Yorkers to file a complaint with her office.

At its peak the AT&T outage saw more than 71,000 customers out of service the morning of Feb. 22. While the outage lasted several hours, service was restored by the afternoon. 

The carrier said the outage was likely caused by a process error, and not a cyberattack. The Federal Communications Commission was investigating the incident. 

Days after the blackout, AT&T said it would apply a $5 credit to impacted accounts.

“We recognize the frustration this outage has caused and know we let many of our customers down,” the carrier said in a statement. “We understand this may have impacted their ability to connect with family, friends, and others. Small business owners may have been impacted, potentially disrupting an essential way they connect with customers.”