AT&T said today its wireless network was back after an outage knocked out cellphone service for its users across the U.S. for hours.

“We have restored wireless service to all our affected customers,” the Dallas-based company said in a statement posted on its website Thursday afternoon. “We sincerely apologize to them.”

Outage tracker Downdetector noted that outages, which began at about 3:30 a.m. ET, peaked at around 73,000 reported incidents. AT&T had more than 58,000 outages around noon ET, in locations including Houston, Atlanta and Chicago. The carrier is the country’s largest, with more than 240 million subscribers.

By 5 p.m. ET, the reports were down to about 1,200.

Cricket Wireless, which is owned by AT&T, had more than 9,000 outages at one point but the reports had also tailed off later in the afternoon. Users of other carriers, including Verizon and T-Mobile, also reported issues but those companies said their networks were operating normally and the problems were likely stemming from customers trying to connect to AT&T users.

Some iPhone users saw SOS messages displayed in the status bar on their cellphones. The message indicates that the device is having trouble connecting to their cellular provider’s network, but it can make emergency calls through other carrier networks, according to Apple Support.

So far, no reason has been given for the outages. But Lee McKnight, an associate professor in the iSchool at Syracuse University, cited the most likely cause of the outage as a cloud misconfiguration, or human error.

“A possible but far less likely outcome is an intentional malicious hack of ATT’s network, but the diffuse pattern of outages across the country suggests something more fundamental,” McKnight said in an emailed statement.

The Federal Communications Commission contacted AT&T about the outage and the Department of Homeland Security and FBI were also looking into it, National Security Council spokesman John Kirby said.

The FBI acknowledged it had been in touch with AT&T. “Should we learn of any malicious activity we will respond accordingly,” the agency said.

The outage also raised concerns on Capitol Hill.

“We are working to assess today’s disruption in order to gain a complete understanding of what went wrong and what can be done to prevent future incidents like this from occurring,” said a statement issued by Cathy McMorris Rodgers, a Washington Republican who chairs the House Energy and Commerce Committee, and Ohio Republican Bob Latta, chair of the Communications and Technology Subcommittee.

Associated Press writers Zeke Miller, Lindsay Whitehurst and Seung Min Kim in Washington, D.C., contributed to this report.