There are a number of cases where young people have committed grave crimes; the recent bombings in Boston, the shootings in Columbine and other places, in all such cases, the crimes have been committed by people who are like old teens or young adults. In a number of such cases, when the crimes have been committed and then a lot of digging into the background is done, there are indicators about how the culprit had made some kind of intentions clear already - there would be postings on forums, there would be Facebook posts that are angry or threatening, and there are questions about how these indicators were not picked up. As a result, there are now tendencies that tend to ensure that such threats on public forums are not taken lightly, and in fact, the people making such threats can be jailed under stiffer terms, such as in this case (link to article):
The Texas teen facing a felony terrorism charge over an alleged threat on Facebook has been released on bail after an anonymous donor posted a $500,000 bond. Justin Carter, 19, had spent five months in prison for posting, during an argument about a video game, what he said was a sarcastic comment about how he was going to "shoot up a kindergarten." "I just think it got taken out of context, and it's been blown out of proportion," Carter told Kate Bolduan on "New Day," CNN's morning show. The brief interview marked Carter's first public comments since he was jailed in February.Just jailing somebody on the basis of their comments (even if the comments are horrific) unless it is a specific threat is dangerous, but at the same time, there is also pressure on enforcement authorities to read these indicators early.