A military wife whose husband was deployed to the Middle East shot her 13-year-old son twice in the head for talking back, authorities said, then returned home and shot her 16-year-old daughter in the face as she studied.

Now she's on trial, and whether she spends life in prison hinges on a key question for jurors: After years of profound mental illness, was she unable to realize what she did was wrong, as her defense attorneys say? Or did she plan the January 2011 killings over at least several days, as prosecutors say, complaining at one point that the three-day wait to buy a gun would "delay the massacre"?

Jury selection began Monday morning in Julie Scheneker's case. She is charged with two counts of first-degree murder.

Those on the jury will see many disturbing images and hear hours of troubling evidence. They will see several sides to Schenecker, a former military linguist and wife of a colonel.

They will see an attentive suburban mom, cheering from the sidelines as her kids played soccer. They will also see a woman who bought a gun the weekend before the killings -- and who complained in a letter that a three-day waiting period would "delay the massacre." They will see the handcuffed inmate, escorted by officers, who walked rigidly into jail following her arrest, her face contorted and eyes empty.

Schenecker, 53, looks older and vastly different than pictures that emerged of her as a blonde, athletic mom in the years before the crime. She sat in the courtroom Monday, flanked by her public defenders, who plan an insanity defense.

Her ex-husband, Parker Schenecker, also was in the courtroom.

Prosecutors are not seeking the death penalty, but if convicted, Schenecker could face life in prison without the possibility of parole. If Schenecker is found not guilty by reason of insanity, the judge could still order her committed to a mental hospital until it is determined she is no longer a danger to others or herself.

According to Tampa Police, Schenecker shot her 13-year-old son, Beau, twice in the head with a .38-caliber pistol in the family car "for talking back." Officers at the scene found the boy still in the passenger seat, with his seat belt on. The report said Schenecker then shot her 16-year-old daughter, Calyx, in the face as the high school cross-country star studied on her computer at their upscale home.

After Schenecker sent her mother a disturbing email, officers went to the house and found her drenched in blood on her back porch. They then found the teens' bodies, a scene that was so troubling that a stress team was called to counsel them.

Julie Schenecker was born in Muscatine, Iowa. There, she was an athlete and that continued in college, where she played volleyball for the University of Northern Iowa.

Schenecker eventually joined the Army, advancing to the rank of sergeant as a Russian linguist. While she was stationed in Munich, Germany, she met and married Parker Schenecker in the late 1980s. She left the service a few years later.

Col. Parker Schenecker was a career military officer and the family moved around. At the time of the killings, he was attached to U.S. Central Command in Tampa. He had been deployed to the Middle East a few weeks before the killings happened. He has since divorced his wife and filed a wrongful death lawsuit against her.

In part, prosecutors declined to pursue the death penalty because they think there is enough evidence to prove Schenecker is mentally ill, but not legally insane.

Under Florida law, the burden of proof will be on Schenecker's lawyers to prove she is insane. They will have to show that she was so mentally ill that she could not understand that killing her children was wrong.

"They have kind of an uphill climb," said Bob Dekle, a University of Florida law professor and retired prosecutor.

It will be especially difficult for Schenecker because she purchased a handgun five days before the shootings, then picked it up after the three-day waiting period and soon used it to kill.

But court documents show that Schenecker has a long history of mental illness. In the early 1990s, she was diagnosed with depression and treated with medication. From 1997 to 2001, she was medicated daily except for when she was pregnant and nursing, records show.

In 2001, she suffered a debilitating bout of depression and was hospitalized for nine months. Documents also show that she was diagnosed with bipolar disorder, schizo-affective disorder and severe depression.

A few months before the killings, Calyx called the police on her mother, saying she had hit her. Police closed the case as there were no marks on the girl. The family was in counseling about a month before the killings.

Dekle points out that "being mentally ill doesn't prove someone is insane," and the two terms are not interchangeable in court.

"Mental illness is a medical concept," he said. "Insanity is a legal concept."