Make an honest and transparent communication to all staff, explaining the situation so that everyone is reassured. If it is something that is not in the hands of the organization, take more drastic measures and form a security brigade. Depends on the emergency

There is only one truth. So establish the true source of information and stick to facts when communicating, not embellished wishful thinking theories or half-baked assumptions. In emergency, our senses are heightened so use language that is not over-emotional. Be clear on what actions you want your audience to take and by what date/ time. Offer a channel through which your audience can ask questions. Maybe a line manager or a dedicated email address? If line managers are suppose to answer additional questions, ensure they are equipped with a script/ resources to answer questions. Above all - timing of your communication is paramount. Allow employees time to take the appropriate actions.