On Monday, Oct 4, Facebook Inc. suffered a devastating outage that shut out many of its 2.7 billion global internet users, idled some of the company’s employees and prompted a public apology from the chief technology officer.

The company’s social-media apps, including the popular social network, photo-sharing app Instagram and messaging service WhatsApp, began to return online for some user on Tuesday morning, more than six hours after the system issue occured. It was one of the longest outaages in recent years. Downdetector, which monitors internet issues, said the Facebook outage was the largest it had seen, with more than 10.6 million reports worldwide.

The reason for the outage was not publicly announced. However, multiple security experts quickly pointed to a Domain Name System (DNS) problem as a possible malefactor. Around 1 pm ET, Cisco's internet analysis division ThousandEyes said on Twitter that its tests indicate the outage is due to an ongoing DNS failure. The DNS translates website names into IP addresses that can be read by a computer. It's often called the "phonebook of the internet."

"We are experiencing networking issues and teams are working as fast as possible to debug and restore as fast as possible." Facebook CTO Mike Schroepfer tweeted, more than four hours after the failure occurred.

Facebook tweeted just after 6:30 pm ET that its apps and services were beginning to work again.

"To the huge community of people and businesses around the world who depend on us: we're sorry," it said. "We've been working hard to restore access to our apps and services and are happy to report they are coming back online now. Thank you for bearing with us."

"I don't know If I've seen an outage like this before from a major internet firm," said Doug Madory, director of internet analysis at network monitoring firm Kentik.