Facebook has disclosed the cause of its outage Monday, one of the worst in its history, and misconfigured routers were to blame.

Facebook, Messenger, Instagram and WhatsApp services went down late Monday morning, and remained down most of the day. The company has revealed the problem in a blog post.

Our engineering teams have learned that configuration changes on the backbone routers that coordinate network traffic between our data centers caused issues that interrupted this communication. This disruption to network traffic had a cascading effect on the way our data centers communicate, bringing our services to a halt.

The company said it is working to learn from its mistake.

People and businesses around the world rely on us everyday to stay connected. We understand the impact outages like these have on people’s lives, and our responsibility to keep people informed about disruptions to our services. We apologize to all those affected, and we’re working to understand more about what happened today so we can continue to make our infrastructure more resilient.