The four major wireless carriers in the US had an outage lasting about five hours in several states last night, and a report from Re/code says it was all caused by a hardware problem in AT&T's network.

Although AT&T, Verizon Wireless, T-Mobile US, and Sprint each operate their own cell towers, in the states where the outage occurred they apparently all acquire backhaul from AT&T's network. Re/code reported that "several telecommunications industry sources" confirmed that AT&T's network caused the outage for all four carriers in parts of Tennessee, Alabama, Kentucky, and Indiana. (Another report said Georgia was affected as well.)

Since AT&T is the dominant provider in the region, "all the major cellular providers use its networks for backhaul—that is, helping transport the data once it leaves their collection of cellular towers," Re/code wrote.

We've contacted all four carriers, and none have confirmed that AT&T was behind the outage. Sprint pinned the blame on one provider but without saying which one. "The outage impacted many other carriers as well and was caused by a third-party vendor," Sprint told Ars last night. "Fortunately, service has been restored and we’re continuing to monitor for stability."

AT&T did confirm to Ars that its network suffered a hardware-related problem and said it was fixed last night. (One report said it was a fiber cut.) AT&T did not tell Ars whether its hardware problem is what caused the outages on Verizon, Sprint, and T-Mobile.

Experts on the business of networking told Ars it's plausible that all the problems originated with AT&T because carriers often have just one source of backhaul outside of major metropolitan areas.

"If they are all using AT&T for backhaul and have no backup, then they are screwed," Dan Rayburn, an analyst at Frost & Sullivan who has written extensively about the business relationships among network providers on his streaming media blog, told Ars. "Question is, why aren't they routing around this?... It's so rare to have an outage like this that they probably don't have a backup [provider], so they can't route around it. They simply make a cost versus quality decision. Outages are rare, so why buy backup and then pay for what you aren't using."

Separately, an industry source told Ars that what allegedly happened last night with AT&T "is highly plausible in many places in the world. It is a dirty little secret that in areas that are less than a major metropole, there is often only one network one can choose from. That network often only has to provide access to its active network under regulated prices. So all traffic into that region is on the same active equipment."

The source, who is an expert on network interconnection, requested anonymity because he works for a telecommunications company in Europe, where AT&T also operates data centers.

"You can imagine that if the incumbent monopolist messes up a software update in its management network, than the whole region can be in trouble, just because the management systems are borked," he also said.

In this case, AT&T said its problem was a "hardware-related network issue."

"Wireless and wireline service has been restored for all customers in parts of the Southeast affected by a hardware-related network issue," AT&T told Ars last night. "Our engineers completed repairs and service is running normally. We apologize for any inconvenience."

Several other reports confirmed that the multi-carrier outage was caused by a single source, without naming AT&T.

"US Cellular blamed two routers with a third-party vendor, which it did not name," WKRN in Nashville reported. "T-Mobile similarly stated earlier Tuesday it was 'in touch with the vendor regarding the source of the issue affecting customers.'"

An earlier report from Re/code quoted Verizon as saying that "the company is working with an unnamed vendor on 'a connectivity issue' affecting service in parts of Tennessee, Alabama, Kentucky, and Indiana."

According to a report by WSMV in Nashville, Tennessee, "Kentucky technology officials said AT&T told them the outage was 'due to a fiber cut in the western part of the state.' An AT&T spokesperson said they cannot confirm those details."