(CNN) -- Steven Kazmierczak had been taking three drugs prescribed for him by his psychiatrist, the Northern Illinois University gunman's girlfriend told CNN.
Jessica Baty said Steven Kazmierczak was irritable but not erratic before his shooting rampage.
Jessica Baty said Tuesday that her boyfriend of two years had been taking Xanax, used to treat anxiety, and Ambien, a sleep agent, as well as the antidepressant Prozac.
Baty said the psychiatrist prescribed the medications, a fact that made her so "nervous" that she tried to persuade Kazmierczak to stop taking one of the drugs.
She said he had stopped taking the antidepressant three weeks before the Valentine's Day rampage on the NIU campus in DeKalb, Illinois, which left five students dead and 16 wounded. He then killed himself.
In an exclusive interview with CNN Sunday, Baty said Kazmierczak had been taking the anti-depressant for obsessive-compulsive tendencies and anxiety caused by school pressures.
She told CNN that, during their two-year courtship, she had never seen him display violent tendencies and she expressed bewilderment over the cause of the rampage. Watch where Kazmierczak turned for gun advice »
"He was anything but a monster," Baty said. "He was probably the nicest, most caring person ever."
Kazmierczak told her he had stopped taking the anti-depressant "because it made him feel like a zombie," she said during the interview Sunday at her parents' house in Wonder Lake, Illinois. "He wasn't acting erratic. He was just a little quicker to get annoyed." Watch girlfriend remember NIU shooter »
She said he had also had problems sleeping.
In her second conversation with CNN, on Tuesday, Baty said Kazmierczak began seeing the psychiatrist shortly after they transferred from NIU to the University of Illinois in Champaign in June 2007.
A psychiatrist not familiar with the details of the case said the three-drug combination was not necessarily either unusual or dangerous.
"It's not terribly unusual to prescribe all three," said Dr. Nada Stotland, professor of psychiatry at Rush Medical College in Chicago and president-elect of the American Psychiatric Association.
Xanax typically has a sedating, calming effect on users, she said.
"If you take a lot of that class of medication, you can be sort of like somebody who is drunk, out of it, but not violent," she said.
A person who had stopped taking it might feel anxious and edgy, she said.
And Ambien is commonly prescribed to overcome sleeping difficulties sometimes attributed to Prozac, she said.
Baty also said that Kazmierczak had been on the computer recently, but she did not know what he was doing and did not ask.
"He was being secretive with his computer," Baty said. "When he would sit on the couch with his laptop he would turn it away from me so I couldn't see what he was looking at."
Baty added that she found Kazmierczak's bank statement on Tuesday, when she returned to their apartment complex for the first time since the shootings.
"He made a big purchase at an ammo store for $143 and some change," Baty said, adding that she thinks he purchased the ammunition at an online store, but she did not know the name or location.
She said he also had made credit card payments and paid the electric bill for their apartment.
Baty disagreed with a report in the Chicago Tribune that said she had given police a different account about Kazmierczak's last days than she gave to CNN.
NIU Police Chief Donald Grady said Baty's statements to CNN contradicted statements she had given to police that her former boyfriend had indeed acted erratically after going off his medication.
"I suppose you could call that being uncooperative," said Grady.
Baty said the comment "upset" her.
"I don't think I ever said he (Kazmierczak) was acting erratic," she told CNN. "If I did, I didn't mean to be contradictory. He was just a little more irritable."
She said she has spoken with DeKalb police every day since the shooting. E-mail to a friend
CNN's Tom Watkins contributed to this story.