Customers of the telecom giant Verizon began reporting cellular outages across the United States starting at noon ET on Wednesday, saying they could not complete calls and had no access to mobile data. Verizon broadband internet customers are also reporting issues. AT&T and T-Mobile customers also began reporting service outages in the same timeframe, though these reports may have been linked to the Verizon outage.

Verizon spokeswoman Christina Moon Ashraf told WIRED in a statement, “We are aware of an issue affecting wireless voice and data services for some customers.

A T-Mobile spokesperson told WIRED in a statement that its service is “operating normally and as expected” but added that T-Mobile’s “customers may not be able to reach someone with Verizon service at this time.” An AT&T spokesperson similarly said its network was “operating normally at this time” and added that the service issues were caused by a different carrier.

The outage tracking site DownDetector shows spikes in reports of service disruptions for all three major US mobile carriers.

Many customers of iOS and Android devices have reported that their phones are in SOS mode, meaning they can only make emergency calls. In practice, however, some reports indicate that the situation may also prevent 911 calls.

Washington, DC official emergency alert channel said one post at 12:57 pm ET that DC’s Office of Unified Communications “is aware of a nationwide Verizon Wireless outage that may affect some users’ ability to connect to 911. If you have an emergency and cannot connect using your Verizon Wireless device, please connect using a device from another carrier, a landline, or go to a police district or fire station to report the emergency.”

Similar incidents happened in the past where an A carrier outage appears to involve several telecomsbecause of the collateral effects if the telecom is not able to route calls and data from customers to other providers.

Syed Rafiul Hussain, a mobile network security researcher at Purdue University in Indiana, speculated that the outage may stem from a server configuration issue in Verizon’s core network. “It’s unlikely to be a coordinated attack against Verizon’s network,” he told WIRED.