Bosco Tjan, a professor of psychology at the University of Southern California, was stabbed to death by a student Friday afternoon.

C. L. Max Nikias, president of the university, sent a message to the campus late Friday in which he said that campus police officers "apprehended the suspect on the scene" and that the suspect was a student. The murder took place in the campus building where Tjan worked.

Meghan Aguilar of the Los Angeles Police Department told the Los Angeles Times that the suspect was a man in his 20s and that the Tjan was the intended victim. “We want to make clear this was not a random act,” Aguilar said. “This victim was targeted by the suspect.”

UPDATE: The suspect, who has been arrested, is David Jonathan Brown, a Ph.D. student in the lab Tjan led.

Tjan was co-director of the Dornsife Cognitive Neuroimaging Center. His research was in perception, vision and vision cognition. His laboratory has received support from the National Eye Institute of the National Institutes of Health and from the National Science Foundation.

On social media, some who knew him mourned his loss.

In a profile of Tjan in the Los Angeles Times, his colleagues described him as caring and constantly going the extra mile to help others with their research. Mara Mather, a professor of gerontology and psychology at Southern California, said, “One of my students, who is now a faculty member, was describing how [Tjan] would take the time to really explain things to her …. He was someone who made it all work and really helped out so many people.” An Associated Press article quoted graduate students who worked with Tjan saying they shared the faculty members' admiration, and they did not know of any conflicts between Tjan and Brown.

Murders of faculty members by students or former students are rare but not unheard-of. In June, a former graduate student shot and killed William S. Klug, an associate professor of mechanical and aerospace engineering at the University of California, Los Angeles. The former student then killed himself.

Last year, a student was charged with threatening to kill a professor at Embry-Riddle University, allegedly over a failing grade. A student at El Camino College was arrested this year after authorities said he sent messages threatening to kill a professor, also over a grade.

Sometimes attacks by a student on a professor have nothing to do with grades or the professor personally.

A Salem State University student was charged in March for stabbing a professor more than 20 times. Officials said there was no connection between the student and the professor, who survived.

Going back over the last 20 years, professors have been killed by students or former students at the Appalachian School of Law, California State University at Los Angeles, San Diego State University and the Universities of Arizona and Arkansas at Fayetteville. The victims of the 2007 mass killings at Virginia Tech -- perpetrated by a student -- included students and faculty members. A professor was among those killed last year in the mass shooting at Umpqua Community College.

In an interview with Inside Higher Ed after a prior murder of a faculty member, Ann Franke, president of Wise Results, a consulting firm that advises colleges on legal issues and risk management, outlined patterns in such violence. She said that the norm in such cases is for the attacker to be male, for the attacks to happen on campus and for the source of the attacker's anger to go well beyond a grade (although that may be a spark).

"These are people who perceive themselves to have serious problems in multiple sectors of their lives," Franke said.