INDIANAPOLIS (WISH/AP) — AT&T says it’s restored wireless service after customers in Indiana and across the U.S. were dealing with cellular network outages on Thursday.

The AT&T outages began around 3:30 a.m. Thursday.

AT&T had more than 58,000 outages around noon ET, in locations including Houston, Atlanta and Chicago. The outages, which began at approximately 3:30 a.m. ET, peaked at around 73,000 reported incidents. The carrier has more than 240 million subscribers, the country’s largest, according to The Associated Press.

The website Downdetector, which monitors phone and internet services nationwide, on Thursday morning that some AT&T customers in central Indiana had reported connectivity issues, lack of signal, and problems with mobile internet.

“Some of our customers are experiencing wireless service interruptions this morning. We are working urgently to restore service to them. We encourage the use of Wi-Fi calling until service is restored,” AT&T said in a statement at 11:15 a.m. Eastern time Thursday.

An updated message was sent at 3:10 p.m.: “We have restored wireless service to all of our affected customers. We sincerely apologize to them. 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 telecommunications carrier, the country’s largest with more than 240 million subscribers, has not said what led to the outage.

Lee McKnight, an associate professor in the iSchool at Syracuse University, believes the most likely cause of the outage is 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.

Cricket Wireless, which is owned by AT&T, had more than 9,000 outages, Downdetector said Thursday.

Verizon had more than 2,000 outages and T-Mobile had more than 1,400 outages. Boost Mobile had about 700 outages.

“Verizon’s network is operating normally. Some customers experienced issues this morning when calling or texting with customers served by another carrier. We are continuing to monitor the situation,” Verizon said.

T-Mobile said that it did not experience an outage.

“Our network is operating normally. Down Detector is likely reflecting challenges our customers were having attempting to connect to users on other networks,” T-Mobile said.

For those still impacted by the outage, Alexander Wyglinski, a professor of electrical and computer engineering at Worcester Polytechnic Institute, says that there is a possible workaround.

“A lot of cell phones do Wi-Fi calling. So, as a potential backup, if you are close to a wi-fi access point, you may be able to use that if you are affected by the network outage,” he said in an emailed statement.