The Masked Truth by Kelley Armstrong

 Riley Vasquez is haunted by the brutal murder of the couple she was babysitting for. Max Cross is suffering under the shadow of a life-altering diagnosis he doesn't dare reveal. The last thing either of them wants is to spend a weekend away at a therapy camp alongside five other teens with "issues". But that's exactly where they are when three masked men burst in to take the group hostage. The building has no windows. The exits are sealed shut. Their phones are gone. And their captors are on a killing spree. Riley and Max know that if they can't get out, they'll be next--but they're about to discover that even escape doesn't equal freedom.

This fast paced thriller is very intense, wrapped in a twisted plot that keeps you on the edge of your seat. It is written from the viewpoints of Riley and Max who let you connect with them emotionally and mentally to understand their hardships, both situational and inside their heads.

It does contain lots of gory violence and a high body count, so is not for the faint of heart and is more suited for older teens.