Centennial, Colorado (CNN) — A psychiatrist who treated James Holmes described him Tuesday as an anxious, anti-social oddball who thought obsessively about killing people in the months before he opened fire in a crowded movie theater in the Denver suburbs, killing 12 and wounding 70.

Dr. Lynne Fenton testified before a packed courtroom that Holmes told her in March 2012 that he had “homicidal thoughts” — as often as three or four times a day. As his treatment progressed, he told her his obsession with killing was only getting worse.

And yet, Fenton told a jury there was little she could do because Holmes never talked about specifics. She couldn’t place him under a psychiatric hold because he never disclosed his intention to kill — even while he was acquiring weapons and “casing” a movie theater. And he never named a target, she testified.

Fenton, who headed the student mental health clinic at the University of Colorado’s Anschutz Medical Campus, was perhaps the most anticipated witness at Holmes’ capital murder trial. Courtroom spectators paid close attention as she testified on the 32nd day of a long and emotionally grueling courtroom drama.

Fenton testified that she treated Holmes from mid-March until June 11, when he left her care upon flunking out of his post-graduate course in neuroscience. He walked out of her office without saying goodbye or shaking her hand, she said.

Just over a month later — on July 20, 2012 — Holmes donned protective gear, tossed a tear gas canister into Theater 9 at the Century 16 multiplex in Aurora and shot into the audience at the midnight premiere of “The Dark Knight Rises.”

He has pleaded not guilty by reason of insanity. If he is convicted of murder, prosecutors intend to seek the death penalty.

Fenton and the university are being sued by relatives of some of the victims, and she had never before spoken publicly about her seven meetings with Holmes.

Just hours before the attack, Holmes mailed Fenton a notebook containing elaborate sketches of the theater, details of his plans, and his rationale for carrying out the shootings. He later told a court-appointed psychiatrist that it was important for people to understand.

The notebook also contained Holmes’ rambling thoughts about life, death and his “broken brain.” He wrote that taking the lives of others would add value to his own life and keep him from committing suicide.

And yet, Fenton testified, Holmes never spoke of taking his own life during his sessions with her. Nor did he talk about feeling manic or depressed or about seeing “flickerings” or other hallucinations, as he did in the notebook. He focused on how social situations caused him great anxiety, mentioned a romantic breakup and repeatedly expressed his hatred toward humanity.

That dislike seemed to extend to her, Fenton said, and he did not volunteer information, responding to questions with terse, one-word answers. She said he was hostile and guarded and “wasn’t very forthcoming.” She suggested he might feel more comfortable with a male therapist, and brought a colleague into their later sessions.

Holmes had contacted the mental health clinic in mid-March, complaining of social anxiety. He was first seen by a social worker who described him as “one of the most anxious people she had ever seen.”

Fenton first saw him on March 21, and tentatively diagnosed him with “social anxiety” and obsessive-compulsive disorder. At their first meeting, Holmes expressed “thoughts about killing people,” but Fenton said she didn’t think he was dangerous. She believed his preoccupation with killing others was a symptom of obsessive-compulsive disorder.

During one session, she said, Holmes described homicide as “a solution to the biological problem,” but added “you can’t kill everyone, so it’s not an effective solution.”

Fenton said she focused on trying to build an “alliance” with Holmes so he would take his medication and keep coming back for counseling. She worried more that he would drop out of treatment, rather than carry out any plan to kill others.

But the diagnosis evolved over several sessions, and she later saw signs of paranoia and psychosis. She spoke with his mother and came to believe his homicidal thoughts were longstanding, dating back to childhood.

After their fourth session on April 17, she noted that he made several odd statements she didn’t understand and wrote in her notes, “psychotic level thinking?” He said his homicidal obsession was “getting worse.”

At that session, she wrote out a prescription using the name “Hughes” rather than “Holmes,” which prompted him to send an email asking “an inconvenience, Fenton?” The memo field contained a strange emoticon, a variation of the letter Q. He later apologized for his “terrible mistake,” stating that the emoticon represented a fist punching the psychiatrist in the eye.

“Violence, is that what you need to hear?” he asked, according to Fenton’s testimony. She said she didn’t fully understand what he meant, but added that he expressed concerned he would be “locked up.”

In May, Holmes spoke of reading about the Unabomber and the “Tylenol terrorist.” She learned that he had failed a course and would leave school. He said he could no longer afford treatment.

He seemed to have a plan, saying he had money in the bank, would get a job and could count on his parents for financial support.

“It sounded like he was functioning well and looking ahead to the future,” Fenton testified. She added that he didn’t seem depressed or angry and expressed “no thoughts of hurting himself.”

By June 11, his last session, Holmes seemed to have developed more of an angry edge, and challenged her, saying she was afraid of him and didn’t like him. Fenton did not call police, but she did get in touch with the campus threat assessment team. She also called Holmes’ mother and learned he had been troubled for many years.

Fenton testified that she asked Holmes if he intended to act on his homicidal impulses, and he said he did not plan to take action. The notebook he mailed her told a different story.

“If the defendant had told you about buying weapons and making plans, would you have acted differently?” prosecutor George Brauchler asked.

“Yes,” the psychiatrist responded, “it would have shown me he had a plan and intent, not just homicidal thoughts.”

Prosecutors said they may finish their case by the end of the week.

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