Reports say the FBI, Department of Homeland Security and other agencies are looking into the cause of Thursday’s cell phone outages. They are looking into whether it may have been part of a cyberattack or just technical difficulties.

AT&T cell phone service was fully restored after being out for most of the day. T-Mobile and Verizon customers also reported issues, but the carriers said their networks did not experience outages and the problems were most likely the result of their customers trying to contact AT&T customers.

Things started going south Thursday morning around 3:30 Central time with AT&T users reporting the most outages according to the website Downdetector.

More than 50-thousand AT&T users were impacted.  The cities experiencing the most outages include Los Angeles, Chicago, Houston and Honolulu.