Emergency 911 services were restored Wednesday night, April 17, in South Dakota, Nebraska, Nevada and Texas following a major outage that left call centers unreachable for many residents. The cause has not been identified but Homeland Security has noted the increasing risks of cyberattacks on digital 911 systems.

In Del Rio, Texas, the issue was traced to a major cellular carrier. Services in Las Vegas and other affected areas have resumed, and all missed emergency calls have been returned.

The South Dakota Department of Public Safety announced a statewide 911 service interruption has been resolved.

“Our emergency system is fully operational and ready to respond promptly to any situation,” the department stated on Facebook. “Your safety is our top priority and we are here to ensure help is just a call away whenever you need it.”

In Las Vegas, a two-hour 911 outage affected both landline and mobile calls before service was restored around 9 p.m., the Las Vegas Metropolitan Police Department announced on social media. During the outage, dispatchers returned calls to residents attempting to reach 911 from mobile phones.

Reuters reports that the Transportation Department’s Office of Emergency Medical Services, the Federal Communications Commission and the Federal Emergency Management Agency did not immediately return requests for comment.