Authorities in schools look for patterns in behavior that can be passed from older to younger siblings when trying to identify students capable of violent acts.

Violence often trickles into schools from disputes and grudges built up over time outside the classroom, in parks and neighborhoods where students spend their time away from school grounds, according to Officer Anna Walker, a school resource officer at John F. Kennedy and James Hubert Blake high schools in Montgomery County.

Some high school students have attended classes together since elementary school and have lived in the same neighborhoods as well, providing ample time to build up long-standing grudges against each other, she said. In some cases, younger siblings are taught by older brothers and sisters about the disagreements, leading to generational feuds.

"I have the beauty of having worked in the middle and elementary schools [in Montgomery County], so I have some firsthand knowledge of these students," Walker said. "Most of the kids I can speak to are ones who really don't have a clear definition of self. They're followers, ones who fall into peer pressures."

Authorities must roam the halls and seek tips from classrooms to identify students whose classmates suspect may be having violent thoughts or may be planning an act of violence, according to Lt. Kim Babcock, supervisor of the Arlington County Police Department's school resource unit.

Repeat offenders are also common clue; a student who has committed a crime once is susceptible to acting out again.

"Those are the ones where we see the same names keep coming across our desk," Walker said.