More than 70,000 AT&T cellular customers reported being unable to connect to service early Thursday morning. While early reports suggested multiple carriers, including Verizon and T-Mobile, seemed to be affected, that appears to be a knock-on effect of a major network going down.

Service monitoring site Downdetector was showing multiple post-paid and pre-paid carriers as having increased outage reports starting at around 4 am Eastern time. An Ars editor in Texas has seen “SOS” on their iPhone since 4:30 am ET and has been unable to make Wi-Fi calls.

AT&T acknowledged the outage to CNBC, telling the network that it was “working urgently to restore service” to customers and that it recommended Wi-Fi calling until service could be restored. Verizon and T-Mobile told CNBC that they suspected their customers were reporting outages when they could not reach customers on AT&T or resellers that use AT&T networks.

Emergency services around the country, including in Chicago and Charlotte, North Carolina, have issued warnings about how such a cellular outage could leave people unable to call 911 during emergencies.

AT&T has not disclosed the source of the outage. An “industry source” told CNN that they suspected the issue was related “to how cellular services hand off calls from one network to the next,” or peering.

This is a developing story. Ars will update this post with new information.

https://arstechnica.com/?p=2005172