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Seung-Hui Cho

Student's attack at California college came without warning

There was no outward warning that a quiet 18-year-old would burst into a university classroom and slash fellow students over a dispute about a study group, investigators say.

Faisal Mohammad had no previous run-ins with police and no clues in his past behavior to suggest that the freshman at University of California, Merced, would try to execute an elaborate plan to kill his classmates as revenge for being kicked out of the group, the Merced County Sheriff's Department said.

In some cases of mass violence, assailants leave signs of their intentions.

Elliot Rodger uploaded a video to YouTube explaining his violent plans before killing six and injuring 14 near the...