This afternoon, AT&T confirmed that it has restored mobile service following a major outage that disrupted its network for customers across the US for much of Thursday. “We sincerely apologize to them,” the carrier said of subscribers in a statement on its website. “Keeping our customers connected remains our top priority, and we are taking steps to ensure our customers do not experience this again in the future.”

The company hasn’t yet offered any explanation as to what caused the outage. ABC News reported that the situation prompted the FBI and Department of Homeland Security to begin “urgently” investigating whether the carrier was the target of a cyberattack. Hopefully, AT&T will offer some clarification on the root cause soon, as there will be many questions about how its network was knocked offline for so long.

The FCC has already said it’s looking into what happened and has been in touch with public safety authorities. Some localities issued emergency safety alerts since the outage prevented 911 calls in certain cases, urging residents not to dial 911 just to see if calls would connect.

AT&T’s main competitors, Verizon and T-Mobile, both put out separate statements on Thursday to emphasize that their networks were operating normally. The severity of mobile service downtime that AT&T experienced today is rare. T-Mobile suffered a significant outage a year ago, but that occurred in the middle of the night, whereas AT&T’s problems ran through much of the business day and proved more disruptive for many.