New York prosecutors said Thursday they are opening an investigation into a wireless network outage earlier this month that left thousands of AT&T customers across the U.S. without cellphone service for roughly 12 hours.

The February 22 outage, which also affected some Consumer Cellular, T-Mobile, UScellular and Verizon subscribers, led to widespread frustration by phone users and briefly disrupted 911 service in some communities. 

“Nationwide outages are not just an inconvenience, they can be dangerous, and it’s critical that we protect consumers when an outage occurs,” New York Attorney General Letitia James said in a statement announcing the probe and inviting consumers in the state whose phone service was interrupted to file a complaint.

AT&T apologized this week for the network disruption and offered a $5 credit to customers. The credit will automatically be applied to their accounts, but AT&T Business, AT&T Prepaid and Cricket customers are ineligible for reimbursement.