Associated Press
This 2013 photo provided by the University of Southern California shows USC Professor Bosco Tjan. A graduate student arrested on suspicion of stabbing Tjan to death was being held on $1 million bail Saturday, Dec. 3, 2016. David Jonathan Brown, a 28-year-old brain and cognitive science student, was arrested in the Friday afternoon attack in the heart of the Los Angeles campus. Tjan was pronounced dead at the scene.

LOS ANGELES — A graduate student arrested on suspicion of stabbing to death the professor who oversaw his work at the University of Southern California was quiet and didn't seem to have many friends but never showed any signs that something may have been wrong, several people who knew him at the university said Saturday.

David Jonathan Brown, a 28-year-old brain and cognitive science student, was arrested on a murder charge in the Friday attack on the Los Angeles campus. His mentor, Bosco Tjan, was killed inside the Seeley G. Mudd building, where he runs an intensive lab that studies vision loss.

Brown, one of just five students who worked in the lab, was arrested without incident, police said, adding that the killing was targeted.

It wasn't clear if Brown, who's being held on a $1 million bond, had obtained an attorney.

In a biography page about the lab and the students involved in it, Brown's is the only one without a detailed description or photograph. His Facebook page is also barren of any photographs and showed he had just a handful of friends.

Nathaniel Kwok, who recently finished working 18 months in the lab, said graduate students like Brown work there 40 to 60 hours a week and develop their own projects required to graduate.

Brown had been working in the lab since around 2013 but took a leave of absence for personal reasons last year that lasted roughly a semester, Kwok said. He didn't know why Brown needed the time off or how close to graduation he was.

"He seemed normal for the most part. He was a little on the reserved side, but he was nice. He was friendly," Kwok said from New York, where he's now in medical school. "There was nothing that ever would have given me some kind of indicator that he would be harboring any kind of sentiment like this."

Kwok said Tjan treated him as a son and that he always loved the professor's honesty, good-natured sarcasm and sharp mind.

Chris Purington, project manager at Tjan's lab, said he last saw Brown the day before the attack, when both he and Tjan attended a PhD student's dissertation.

"It didn't seem like anything was wrong. I didn't see this coming," Purington said. "When I heard a grad student did it I really expected it to be somebody I didn't know."

He described Brown as businesslike.

Kilho Shin, who also worked in Tjan's lab, said Brown was quiet and seemed satisfied with Tjan's oversight.

"I don't know what exactly happened between them. But as far as I know, Bosco likes David's work and David also seemed to be satisfied with his supervising," Shin said. "Their conversation on research was healthy and constructive."

Irving Biederman, a neuroscience professor who was Brown's adviser for a couple months, said he had heard from colleagues that Brown was not doing well in school, though he said he didn't have first-hand knowledge of the student's work.

As for Tjan, he said he knew the professor back when he was a student at the University of Minnesota in 1988 and that his death is a huge blow.

"This is the first time I've heard of any student having a problem with Bosco; he was universally admired and loved," Biederman said. "This is almost unfathomable."

Biederman said Tjan was gentle with students, though he was generous with constructive criticism.

Purington said he never heard of anyone having a problem with Tjan, a married father of one son listed in public records as 50 years old.

"He was somebody who really cared about people. I know he cared about me," Purington said through tears.

David Clewett, a student in Tjan's brain-imaging course, said his teacher was "a brilliant scientist and an exceptional person who was always kind, generous with his time, and genuinely cared about his students and colleagues."

Student Kingson Man said: "He was the one we all went to with the really hard, really technical questions. He was a great scientist and role model."

Tjan joined USC in 2001, taught in the Dornsife College of Letters, Arts and Sciences, and served as co-director of the Dornsife Cognitive Neuroimaging Center.

The stabbing comes six months after a well-loved professor was fatally shot on the nearby UCLA campus. Authorities believe former student Mainak Sarkar killed his estranged wife in a Minneapolis suburb before driving across the country to Los Angeles and fatally shooting engineering professor William Klug and killing himself on June 1. Klug had helped Sarkar earn his engineering Ph.D. in 2013.

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