What to say in the first hour of a bad day

In the first hour you rarely know everything, and that is fine (sample briefing). A holding statement says what you know, what you are doing and when you will say more.
Three or four sentences are enough. Show that you care about the people affected before you mention the organisation, and never guess at causes or numbers.
Then tell your own staff before the media. They will be asked by friends and neighbours, and they deserve to hear it from you.