The FCC’s Public Safety and Homeland Security Bureau in an update given to commissioners Thursday said more than 12,000 people were unable to reach 911 directly during an AT&T Mobility outage on March 8.

According to Bureau Chief Lisa Fowlkes, AT&T indicated that on a normal day it fields an average of 44,000 voice-over-LTE (VoLTE) calls nationwide. But during its March 8 outage of VoLTE 911 service, approximately 12,600 unique callers were unable to reach the staple emergency line. Fowlkes noted, though, that a “small subset” of calls were answered by a backup call center and routed to first responders.

The outage lasted for approximately five hours, and spanned the southeast, central, and parts of the northeast regions of the country. Fawlkes noted a second outage that occurred on March 11, attributed to a hardware failure, is being investigated separately.

The Bureau’s preliminary investigation into the March 8 incident indicated AT&T reconfigured connections in the network impacting the routing of 911 calls for VoLTE subscribers, and the subsequent failure of automated call routing was caused by these changes. While a manual routing backup system was activated, the volume of calls proved to be too large for the fallback to handle, thus resulting in a large number of blocked calls. Some customers trying to reach emergency services heard fast busy signals after dialing, Fawlkes said.

The slack was picked up by local public safety officials, who Fawlkes said were notified of the outage either via AT&T’s social media accounts or directly via email. Some officials had to call AT&T themselves to confirm 911 service was down. Once notified, these public safety officials took steps to notify the public of the outage via “mass notification services, social media, and television and radio broadcasts,” and provided alternative 10-digit emergency numbers.

To get more information on the cause, effects, and implications of the outage, Fawlkes said the Bureau has put out a public notice requesting comments from outside stakeholders. She said such a call for public comment is not unusual, and was similarly done as part of the investigation into the 2012 derecho storm.

Fawlkes said AT&T has been fully cooperative with the investigation.