The highly anticipated murder trial of Michele Williams began today in Tarrant County, and for the first time in the three-year investigation, prosecutors introduced a new theory of exactly what led to Greg Williams' death.

Greg was shot and killed while lying in bed at the couple's lavish Keller home in 2011. His wife, Michele, is accused of murder and tampering with evidence.

During opening arguments, assistant district attorney Jack Strickland said that a wound Michele received on her cheek came from a fight with Greg shortly before he was killed. "Greg's close-to-last act was to strike Michele," Strickland said.

Michele has previously told authorities that an intruder smacked her in the face with a wrench and, alternately, that she hit herself in the face to create a more convincing cover story for her husband's suicide.

Strickland submitted that both theories were untrue, and that Michele and Greg had fought over money in the hours before his death. Documents previously reviewed by CultureMap show that the financing for the couple's new home — which they were scheduled to close on the day Greg was killed — had been falsified.

"The motive was money. Tried-and-true, age-old money — greed," Strickland said.

Defense attorney Clay Graham argued that because Greg ran his own successful computer business, he was worth more to Michele alive than dead. Graham characterized Greg's death as a suicide and painted Greg as depressed and volatile, pointing out his habitual steroid use and the recent deaths of his brother-in-law and grandmother.

With his hand on her shoulder, Graham said Michele "panicked" when she cleaned his hands with bleach and rearranged the crime scene.

"Michele reacted to what she was faced with," Graham said, adding, "She did what any mature mother would do. She moved to protect her child from the sight of her father and the question of why."

Michele has frequently changed her story about what happened the night of Greg's death, flip-flopping between a masked intruder and a suicide that she sought to cover up in order to spare their 4-year-old daughter from knowing that her dad had killed himself.

"She acted to protect her child ... because she couldn't protect Greg Williams from himself," Graham said.

Testimony in the murder trial is expected to last about a week.

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New to the story? Catch up on the Michele Williams case here:

October 31, 2013: The making of the Keller black widow: Did Michele Williams get away with murder?

May 15, 2014: Accused murderess Michele Williams gets bond revoked amid fears she'd skip town

January 30, 2014: Keller black widow Michele Williams lands back in Tarrant County jail

February 10, 2014: Michele Williams rejects plea deal and opts for murder trial in husband's death

March 6, 2014: Keller black widow Michele Williams worked at strip club while out on bond

May 2, 2014: Peter Van Sant dishes on what made Michele Williams case so intriguing to 48 Hours

May 28, 2014: Couple offers to help post bond for accused murderer Michele Williams