According to Dr. Sharon Stephan, co-director of the Center for School Mental Health in Baltimore, mental illness often begins in youths by age 14, but "with most severe mental illness and psychosis, the first episodes most often occur between the ages of 18 and 25."

Big life transitions — such as Green's move from undergraduate to graduate studies — can cause problems, she said.

Many experts are pushing for work on earlier identification of and intervention for young people with mental illness, so that violent episodes are prevented, Stephan said. Gov. Martin O'Malley has proposed establishing a center to study how to provide such interventions in the state.

According to Limansky, the university police spokesman, a student supervisor of the student police aide program called Green in early September after he had failed to appear for two scheduled shifts following his orientation into the program, and asked him why he hadn't showed.

"We said, 'What's the deal?' and he said, 'Oh, I didn't feel good,'" Limansky said. The supervisor asked Green for a doctor's note, but Green "decided it wasn't worth his while" and never provided one, which led to his firing, Limansky said.

Since a recent robbery on campus and other crimes in the College Park area, university police have been stepping up night patrols and actively showing their presence, Limansky said. Patrol officers recently started to keep lights atop their vehicles flashing at all times, Limansky said.

Since the shootings, university officers also have been focusing on having "more face time" with students, reading their body language, he said.

"We're walking up to students, talking with them, saying, 'Hey, did you hear what happened? Is there anything we can do for you?'" Limansky said.