On Jan. 23, Columbine High School, where two former students killed 13 people in 1999, went on alert after receiving threatening phone calls. Authorities applied the alert to several other schools in the area, including Standley Lake, according to the Associated Press.
On Dec. 13, student gunman Karl Pierson, 17, fatally shot Claire Davis, a 17-year-old classmate at Arapahoe High School in nearby Centennial, Colo., before killing himself in the school's library. Pierson reportedly had threatened a teacher and librarian who had disciplined him last year and allegedly was seeking that teacher when he entered the school, investigators have said.
In Columbia, Md., on Jan. 25, a 19-year old man opened fire with a shotgun in a mall, killing two employees at a skateboard store. He then committed suicide. Two days later, a 31-year-old man was arrested at another Maryland mall for threatening to kill employees of a luggage store.
"It's called the bandwagon effect," said James Janik, chief psychologist at the Cook County (Ill.) Juvenile Detention Center. "Any kind of behavior that happens around you, you are apt to increase."