Slack, the messaging platform used by millions of people around the world, was experiencing what the company called an “outage” on Thursday.

Reports that the platform was not working began flooding into Down Detector shortly before 5 a.m. Eastern time.

“Users may be experiencing trouble with sending messages in Slack,” the company said on its website just after 5:30 a.m. “We’re investigating and will let you know as soon as we know more. We appreciate your patience in the meantime.”

Slack has become an essential workplace tool, with more than 100,000 companies using the platform.

This is a developing story.