Now a professor who had Thompson in his class said he told a university administrator 10 months ago of his concerns about the mental health of the 20-year-old.
UCLA history professor Stephen Frank said he met the suspect in his Western civilization class late last year.
Frank told the Los Angeles Times he became worried when Thompson sent several e-mails complaining that classmates sitting near him had made offensive comments to him while he was taking a written exam. In one e-mail, Thompson also accused Frank of taunting him.
"I believe I heard you, Professor Frank, say that I was 'troubled' and 'crazy' among other things," Thompson wrote in one of the e-mails. "My outrage at this situation coupled with the pressure of the very weighted examination dulled my concentration and detracted from my performance."
Frank said he was present during the entire exam and saw nothing to support Thompson's complaints. Several teaching assistants also said they saw nothing unusual, Frank said.
Frank said other professors have reported similar exchanges with Thompson, who complained he was the constant target of taunts from students in dorms, dining areas and the library.
"My concern was in the context of other violent incidents on campuses around the country," Frank said.
But a university official told Frank they couldn't order Thompson to seek psychological services.
UCLA spokeswoman Carol Stogsdill said Saturday that university officials were forbidden by privacy laws from discussing how the school may have reacted to any complaints about Thompson's behavior.
All this comes as investigators are piecing together what led to the brutal slashing.
Thompson, who relative said left Belize to attend the university, remained in jail Friday on $1 million bail and was scheduled for arraignment Tuesday. Authorities did not know if he had obtained an attorney. He was arrested shortly after the attack Thursday and booked for investigation of attemped murder.
Stunned University of California, Los Angeles, students watched in horror as blood gushed from the student's neck, forming a puddle on the chemistry lab floor as instructors struggled to stanch the 20-year-old woman's wound.
"Her eyes rolled back in her head, I called out her name and told her to stay with me. She wasn't really responding. I think she could hear me," said chemistry lecturer Stacie Nakamoto.
The victim, who Nakamoto and police would not identify, went to the hospital in critical condition. On Friday, her family released a statement saying she was showing signs of improvement and was expected to recover.
The relationship between Thompson and the victim wasn't clear, but Detective Mike Pelletier said they were not romantically involved.
Cyril Baida, a teaching assistant who was working in a lab across the hall, said he did not know the victim or the suspect but was told that they were lab partners or had worked together in a small group on projects in their lab section.
Immediately after the attack, Thompson walked into the student information center three floors below.
"He was very calm and said he had stabbed someone," said Carol Verduzco, an administrative assistant who works in the office.
She said she asked if he was joking and then she called the police. Thompson waited in a chair for the few minutes it took police to arrive.
"I was in shock," Verduzco said, noting she saw no blood on Thompson.
A knife was recovered at the scene, a laboratory on the sixth floor of the Young Hall chemistry and biochemistry department in the heart of the university on the west side of Los Angeles.
UCLA spokesman Phil Hampton said he could not provide details of the attack, adding: "Everything is still under investigation."
Thompson's relatives said he is an only child who left his mother's home in Belize two years ago to attend UCLA. His second cousin, 17-year-old Akilah Williams, said she was skeptical that Thompson was capable of the attack.
"He cares about what people think about him too much," Williams told the Los Angeles Times. "He gets stressed out but that doesn't mean he'd do something crazy."