Carmela Dela Rosa calmly went to the edge of a five-story walkway at Tysons Corner Center last November and sent her two-year-old granddaughter plunging over the edge, authorities allege.

The horrific killing during a routine family outing shocked Dela Rosa’s relatives and baffled neighbors, who described the Fairfax City-area woman as a warm and loving grandmother. Police sources have said nothing appeared to have precipitated the attack.

On Monday morning, Dela Rosa, 51, who has been charged with murder in the death of Angelyn Ogdoc, pleaded not guilty by reason of insanity in a Fairfax County courtroom. A jury was being selected for a trial that is expected to last about ten days.

The case will revolve around a single question: What was Dela Rosa’s mental state at the time Angelyn was tossed over the railing?

Deputy Public Defender Dawn Butorac has said that Dela Rosa has a history of mental health issues. The defense has retained a psychologist who has evaluated Dela Rosa and has subpoenaed 18 witnesses, including numerous mental health experts.

Fairfax County Commonwealth’s Attorney Ray Morrogh tapped a high-profile clinical psychologist, who has cast doubt on insanity pleas at previous trials, to evaluate Dela Rosa. The defense unsuccessfully argued against Dr. Stanton Samenow’s appointment in preliminary motions.

The defense has subpoenaed six witnesses, including Dela Rosa’s husband, Leandro.

On Monday in court, Dela Rosa’s once long hair had been cut short. She wore a black pant suit.

Court filings in the case have revealed fresh details about Dela Rosa in the months and years leading up to her granddaughter’s death.

Fairfax County police officers said Kathlyn Ogdoc, Dela Rosa’s daughter and Angelyn’s mother, told them Dela Rosa was unstable and had been depressed since 1998, when her father died, according to court documents.

Ogdoc also said Dela Rosa tried to commit suicide several times, including attempting to drive her car off a cliff in the Shenandoah area and overdosing on an undisclosed drug, court documents said.

In an April 2009 letter to her daughter in the court file, Dela Rosa said she was hurt by Kathlyn’s direction in life.

“I feel very betrayed and disappointed with what you had done back in 2007,” Dela Rosa wrote. “I wanted you to enjoy your 20s without any responsibility of a husband and child.”

Kathlyn Ogdoc testified at a dramatic preliminary hearing in February that she and her family were walking across the skyway between the Tysons Corner Center and a a parking garage on the evening of Nov. 29, 2010, when she saw Dela Rosa scoop up Angelyn.

Ogdoc, who was choking back tears during her testimony, said she took a few more steps and looked back again and saw her mother at the walkway railing "drawing her hands back.”

"I yelled, 'Mom!'" Ogdoc said and sprinted down the stairs with her brother and father to the street below. Angelyn had hit the pavement, but was still alive, Ogdoc testified. She recalled glancing up at her mother, who had crossed her arms on the railing, placed her chin on them and was glancing down placidly.

Angelyn was rushed to Inova Fairfax Hospital, where she died hours later. Ogdoc did not actually witness her mother drop Angelyn over the side, but sources said a mall surveillance camera captured the event. The footage has not been played in court.

Officer Anthony Stancampiano, who was dispatched to the scene, testified at the hearing that he went up to Dela Rosa, after seeing her on the walkway talking with a mall security guard.

"I walked up to her and asked if she was the one involved in this," Stancampiano said. "She said, 'Yes, I did it. I threw the baby off.' "

If Dela Rosa is found guilty, she could face a sentence of 20 years to life in prison.

This post has been updated.