CAMBRIDGE, Mass. (AP) — A man on trial for killing seven co-workers tripled his dosage of Prozac before the shootings, a move that may have heightened his rage and sparked the shooting spree, a defense psychiatrist testified Wednesday.

Dr. Anthony Joseph said Michael McDermott suffers from paranoid schizophrenia and other mental disorders that made him unable to understand that what he was doing was wrong when he opened fire at Edgewater Technology on Dec. 26, 2000.

Prosecutors contend McDermott killed his colleagues because he was angry over the company's decision to withhold some of his salary to pay back taxes.

But Joseph said McDermott told him that he had increased his dosage of Prozac by Dec. 1, first from 70 milligrams per day to 140 milligrams, and then to 210 milligrams. Joseph said McDermott increased the dosage without his doctor's permission or advice.

"It's very possible that Prozac is the final piece of the puzzle that explains the level of rage and anger that allowed the killings to occur," Joseph said.

Although Prozac acts as an antidepressant, potential side effects include restlessness, agitation, psychosis, rage, anger and violence.

Joseph acknowledged he could not say to "a reasonable degree of medical certainty" what effect the increased dosage had on McDermott.

On Thursday, prosecutors planned to cross-examine Joseph.

Prosecutors also plan to call witnesses to support their theory that McDermott concocted an elaborate tale to make himself look insane to the jury.

On the witness stand last week, McDermott, a 43-year-old software engineer, said he believed he killed Nazis – not his co-workers.

He said St. Michael the Archangel appeared to him before the killings and told him he could prevent the Holocaust and earn a soul if he traveled back in time to 1940 and killed Adolf Hitler and six German generals.

In testimony Wednesday, a defense psychologist said that a psychological test he gave McDermott in November indicates he is not feigning symptoms of mental illness.

Dr. Anthony Kalinowski said the test results showed an eccentric, depressed and angry man who blames others for his problems.

McDermott has said he researched symptoms of mental illness for years so he could appear sane to doctors and so that he could get the types of antidepressants he preferred.

Under cross-examination, Kalinowski acknowledged that McDermott's knowledge of the test could have improved his ability to manipulate the results.