NEW YORK (AP) — Some Verizon customers in the U.S. were hit by a service outage Monday.

The problem appeared to knock out cell phone service for tens of thousands of Verizon users. Data from outage tracker DownDetector shows that the number of reports reached the 100,000 mark shortly after 11 a.m. ET — and while that number dropped significantly, nearly 48,000 were still experiencing issues around 4 p.m. ET.

Frustrated users online said they were without service in many parts of the country, including the southeastern US, where residents are still reeling from the aftermath of Hurricane Helene. But the outage is not limited to one region. DownDetector’s map also showed many reports coming from the West Coast, Midwest and Northeast.

In an update on social media platform The company added that it had technicians investigating the issue and was “working quickly to identify the service and resolve the issue.”

The Federal Communications Commission also acknowledged the outage later in the day, but provided no further details other than to say it wanted to determine the cause.

Some Verizon iPhone customers were seeing SOS messages displayed in the status bar on their cell phones. The message indicates that the device is having trouble connecting to the mobile carrier’s network, but can make emergency calls over other carriers’ networks.

During these types of cell service outages, an alternative is to connect to nearby Wi-Fi. Wi-Fi calling is also a built-in feature on most Android devices and iPhones and can be enabled through the phone settings.