Edit and revise this memo carefully as it will likely be the only hard evidence of your levelheadedness during the situation. As you gather facts, be aware of your team's sensitivities. They know something ugly has happened and will likely be concerned for their own jobs. Be cool and levelheaded. This is not easy when one of your direct reports admits to making a boneheaded mistake, but it is absolutely essential.
Opportunists exist in every organization, and they are easy to identify during a tough situation.
They're the ones pointing fingers. Protect yourself and your team. Don't let these few bad apples upset your sound judgment. When you know all the facts, create, communicate and execute a plan to fix the problem and ensure it will not happen again. Knowing about a problem and not acting to correct it is asking to be fired.