HOUSTON -- The star witness in the capital murder trial of Andrea Pia Yates, 37, surprised court watchers Friday with a non-committal answer about Yates' competence.

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Renowned forensic psychiatrist Park Dietz was the state's star witness. He interviewed Yates last November.

Prosecutors are trying to prove that Yates was legally sane when she drowned her five children in their southeast Houston home on June 20.

When prosecutor Joe Owmby asked Dietz about the issue of sanity, Dietz said he had no opinion.

Dietz testified that Yates knew her actions were wrong in the eyes of the law, the eyes of society, and the eyes of God, but may have believed it was in the best interests of the children.

"The question I asked him was the ultimate question in the case and I asked him an incorrect question and I should not have asked him the question because he doesn't give the ultimate answer in the case. The jury does," Owmby said.

Defense attorney George Parnham was asked if Dietz's answer was a blow to the prosecution.

"I don't know if it's a blow or not. I thought his testimony was credible. I thought he made an excellent witness," Parnham said.

Dietz testified that he believed Yates was mentally ill at the time of the drownings, but didn't become fully psychotic or believe that Satan was living in her until she was booked into jail and put on suicide watch.

Dietz also showed clips from a jailhouse interview with Yates in which Yates said she believed what she did was right, both before and during the drownings.

In the video, Yates also described drowning her children one by one, saying, "While I was filling the tub, Paul came in. He said, 'Mommy are you taking a bath today?'" Then, sobbing, Yates described Noah struggling with her saying, "he came up out of the bath water and said something. I didn't know what it was. If he was saying, 'I'm sorry' or what."

Dietz also testified that if Yates thought drowning her children would send them to heaven, why didn't she comfort them and tell them that were going to be with Jesus?

The defense will cross-examine Dietz on Saturday, when a special weekend session of court will be held, beginning at 9:30 a.m.

Jurors are expected to get the case in the middle of next week.

Yates, 37, has pleaded not guilty by reason of insanity to capital murder charges for drowning Noah, 5-year-old John and 6-month-old Mary at their Houston home. Charges could be filed later in the deaths of Paul, 3, and Luke, 2.

If jurors convict her of capital murder, Yates faces life in prison or the death penalty. If she is found not guilty by reason of insanity, the court will have to determine whether to commit her or set her free.