Police are warning Hudson Valley teens to watch what they post on social networking websites because authorities are watching.

The warning comes after police arrested a Walden teen for allegedly assaulting and stalking two fellow students at Valley Central High School. Police say Facebook helped them make an arrest. "I don't think [students] have an understanding how detrimental it can be," says Michelle Kozlowski, a resource officer at Warwick Valley Middle School.

Kozlowski says she checks on Facebook for cyber-bullying and other reports that could lead to arrests of teenagers.

Police arrested the Walden teen after a fight allegedly broke out between her and two classmates. The teen then followed the two others home, but left soon after, police say.