The Federal Communications Commission is investigating an alarming 911 outage that affected AT&T Wireless customers nationwide Wednesday. Police departments in various cities reported the outage, with customers unable to dial 911 from their cellphones. Police took to social media to offer alternative, non-emergency phone numbers in case of emergency, though that meant dispatchers were unable to see callers’ locations. Although the issue was resolved late Wednesday, it was unclear what caused the widespread outage. Ajit Pai, the chairman of the FCC, announced an investigation into the incident on Twitter. “The FCC will investigate the root cause of the outage and its impact,” he wrote.