Facebook said in a blog post Monday night that the six-hour outage that took it offline, along with Instagram, Messenger, Whatsapp, and OculusVR, was the result of a configuration change to its routers — not of a hack or attempt to get at user data. While the initial explanation didn’t really explain things, a subsequent blog post on Tuesday went into way more detail, saying that the outage was due to a routine maintenance mistake that basically disconnected Facebook’s datacenters from the internet.

CEO Mark Zuckerberg posted an apology Monday evening, saying the platforms were coming back online. “Sorry for the disruption today — I know how much you rely on our services to stay connected with the people you care about.”

The outage began around 11:40AM ET Monday and led to widespread problems for the company. It was Facebook’s worst outage since 2019, when the site was down for more than 24 hours. Employees were unable to connect with each other on company message boards, and some told The Verge they were using work-provided Outlook email accounts to communicate.

The problems cascaded to affect the servers that advertise Facebook’s DNS and BGP information. That failure wiped out the DNS routing information that Facebook needs to allow other networks to find its sites.

Facebook’s outage came a day before whistleblower Frances Haugen was set to testify before Congress about her experiences at the company. Haugen, a former Facebook product manager who worked on its Civic Integrity group, provided a trove of internal Facebook documents to reporters at the Wall Street Journal. She told 60 Minutes on Sunday that Facebook “pays for its profits with our safety.”

Update October 5th, 2:55PM ET: Added information from Facebook’s Tuesday engineering post.