A、The gunman has mental disease. B、The gunman is dissatisfied with the university. C、The gunman is dissatisfied with the lecture

The shooting occurred Thursday in a lecture hall at Northern Illinois University in DeKalb, Illinois, north of Chicago. The gunman entered the hall dressed in black and carrying a shotgun and three handguns and began firing as students sought cover under desks. One student who escaped unharmed told reporters his life had changed as a result of what he experienced. Police say the gunman was Steven Kazmierczak, 27, a former graduate student in Sociology at the school.
Police say he purchased the shotgun and two of the handguns on Saturday, indicating he may have planned the attack in advance, but authorities have not determined a motive for the shooting. Kazmierczak had no criminal record and no history of mental illness. Under Illinois state law he would not have been able to purchase a weapon legally if there had been a record of arrests or mental problems. But police say he stopped taking an unspecified medication recently and his behavior became erratic.
1.  What is this news item mainly about?
2.  What is the reason of the shooting?