That is true. And one researcher who wrote a book on the topic says there are patterns. 99 to 100% of them also tell someone of their violent plans before going on their rampage.
Typically, they are 16 to 20 years old and kill close family member(s) before doing the mass shooting. They want to save them from being embarrassed and shamed afterwards.
They're going to use Sandy Hook as an excuse for stricter gun control, imo.
The man who's a carpenter and drives around putting up white crosses for all the victims of shootings all over the country carries a 45 in his truck. He says all teachers should be armed. At the very least, I think schools should have armed security guards who have undergone stringent background checks and mental health screening.