The student who stabbed four people on the campus of UC Merced was identified Thursday as 18-year-old Faisal Mohammad from Santa Clara, Calif., the university confirms to Fox News. "He had a smile on his face, he was having fun," a construction worker who helped stop the attack told CBS 47. Campus police shot and killed Mohammad, a freshman computer science & engineering major, after Wednesday's attack. Police detonated his backpack and are testing a substance inside, KGO reports. The four victims are expected to survive. Authorities are investigating a motive.