Perhaps it means that their HANDLERS are in prison / incapacitated / etc., and/or there's a break in their communications.
We pretty much know that none of those people mentioned write their actual tweets. It's all controlled by staff, and it's likely those people get their marching orders from higher up, especially for stories like this one.
Why in the world would those people be silent when it's so simple to tweet a common sense message praying for the victims in the school shooting?