Police allege that on April 15, 2005, Andrea Petrosky drowned her son Garrett in a bathtub after strangling him, then called 911 and confessed. Charged with capital murder, the 39-year-old former Roanoke resident is facing the death penalty.

Petrosky defense counsel Jay Finch with the Capital Defender Office for Western Virginia confirmed this week that her attorneys have filed notice of intent to put on an insanity defense. Finch declined to comment further.

 Bristol Commonwealth’s Attorney Jerry Wolfe, the prosecutor in the case, did not return a phone call seeking comment Wednesday.

The insanity defense is raised in less than 1 percent of felony cases nationwide, said University of Virginia law professor Richard Bonnie. Most acquittals by reason of insanity happen because prosecution and defense reach an agreement before a case goes to trial, he said.

If an insanity plea actually goes to trial, the defense has an uphill battle, Bonnie said.

Petrosky’s case resembles the high-profile case of Andrea Yates, a Texas woman who drowned her five children in a bathtub in 2001.

Yates, too, faced the death penalty. A jury sentenced Yates to life in prison, but the verdict was overturned. On Wednesday, a new jury found Yates not guilty by reason of insanity.

In the Yates case, the prosecution and defense agreed that Yates was mentally ill and that she killed her children as a result of her delusions, but they disagreed over whether she was legally insane, Bonnie said.

Petrosky’s defense attorneys received the results of a mental evaluation before announcing their intent to pursue an insanity plea. Prosecutors are also seeking an evaluation. The future of the case will likely depend on whether the psychological experts for the two sides agree, Bonnie said.

According to statistics kept by the Virginia Department of Mental Health, Mental Retardation and Substance Abuse Services, an average of 35 people a year are acquitted of a crime by reason of insanity in Virginia.

Prosecutors and police have not commented as to whether they know of any motive for the slaying. Petrosky’s trial is set to begin Oct. 16.

When she was charged, she became only the third person in Virginia to be prosecuted under “Annie’s Law,” a relatively new capital murder statute that makes it a capital crime for a person 21 or older to kill a child younger than 14.

Petrosky and her family lived in Roanoke for several years before moving to Bristol in 2004.