North Korea's Internet was restored Tuesday local time, after an outage that lasted for 9 hours and 31 minutes, according to a company that monitors Internet performance. Dyn Research had earlier reported that the four networks in the country were down.The cause of the outage is not known, but led to speculation that the U.S. may have retaliated for North Korea's alleged role in the hack of Sony Pictures in late November.

The U.S. Federal Bureau of Investigation said Friday it held North Korea responsible for the attack, which came ahead of the planned release by Sony of a comedy movie about a plot to assassinate the country's leader Kim Jong Un.

President Barack Obama said soon after that at a news conference that the U.S. would respond proportionally to the attack "in a place and time and manner we choose.

"All four North Korean prefixes have been restored to service at 01:46 UTC, after a national outage of nine and a half hours. Traffic is routing through China Unicom, just as before," Dyn said in an update. It also tweeted that North Korean-hosted websites had returned.

The company had earlier reported that North Korea went off the Internet at 16:15 UTC after more than 24 hours of sustained weekend instability, but said such long outages were not without precedent in the country.

"A long pattern of up-and-down connectivity, followed by a total outage, seems consistent with a fragile network under external attack," wrote Jim Cowie, Dyn's chief scientist in a blog post. "But it's also consistent with more common causes, such as power problems."

North Korea is dependent on a single international provider, China Unicom.