Time Warner Cable says a problem that occurred during routine maintenance caused a nationwide outage of its Internet service for hours on Wednesday morning.

The company says it is still investigating the cause of the problem, which occurred with its Internet backbone, the paths that local or regional networks connect to in order to carry data long distances.

The company says the problem affected all of its markets and started at 4:30 a.m. and was largely restored by 6 a.m., and updates continue to bring all customers back online. The outage sparked widespread complaints on social networks.

Time Warner Cable, which is being acquired by rival cable company Comcast, has about 11 million subscribers nationwide.