CENTENNIAL, Colo. — Colorado theater shooter James Holmes' ex-girlfriend said she asked him to see a therapist after he mentioned having thoughts about killing people months before the attack, but she said his thoughts "seemed very philosophical" and not a concrete threat.

Gargi Datta testified Thursday about talks and online chats she had with Holmes when they were dating during graduate school at the University of Colorado-Denver.

In early 2012, Datta said, she and another student asked Holmes to see a therapist, and Holmes said he already was.

Holmes also expressed frustration with his field of study — neuroscience — saying science had become too social, Datta said.

"He recognized that he had issues interacting with other people in public, and he thought that they were judging him," she said.

Holmes has pleaded not guilty by reason of insanity in the July 20, 2012, attack at a suburban Denver movie theater that killed 12 people and injured 70.

On Wednesday, she testified that the two began dating during their first semester in the fall of 2011. By February 2012, she said, she did not want anything more than a casual relationship, and she broke it off.

Their first date was a horror film festival and they grew closer after that, sharing nights together at home watching movies or playing board games with friends.

Their breakup was a catalyst to the shooting five months later, District Attorney George Brauchler said. He said she was Holmes' first romantic relationship.

In brief questioning before court adjourned Wednesday, Datta said she met Holmes in 2011 at the graduate school outside Denver. She described him as a bright but shy neuroscience student at the time.

"He wouldn't go up and interact with people," Datta said, adding that Holmes was more talkative when he was alone with her.

They started spending time together after she invited him to a study session. Holmes emailed his phone number to her and said that if she texted him, "I will tell you an amazingly, best-ever world's greatest knock-knock joke."

Datta said they were spending one or two nights a week together in the months before the shooting. After they broke up, Holmes never revealed to her that he was amassing an arsenal of weapons and body armor in meticulous preparation for the attack, prosecutors said.

Holmes has pleaded not guilty by reason of insanity in the shooting. Prosecutors contend Holmes was sane, and they are seeking the death penalty.

In court, Datta never looked directly at Holmes and repeatedly referred to him as "the defendant" rather than his name. Holmes swayed in his chair as she spoke but had no other visible reaction.

In a video played earlier in the trial, Holmes said he fell in love with Datta. He had written about her in emails to his parents, saying he had cooked dinner for her on Valentine's Day. The two played board games together with friends, watched movies at home, shared dinners and were in the same stressful classes, Datta said.

But Datta said she wanted to end the relationship in January 2012 when she returned from winter break.

"I had already told him in the start that it was a casual relationship," she testified. "I didn't feel I was getting closer to him. I think he liked me more than I liked him."