Prosecuting attorney Jonathan Mosher gives his opening statement in the murder trial of David Watson, Wednesday, Oct. 5, 2016 at Pima County Superior Court in Tucson, Ariz. The trial against a former Arizona fire captain who rose through the ranks as police suspected him in three murders concluded its first day with damming testimony from the defendant’s one-time alibi. David Watson is accused of killing his ex-wife, Linda Watson, in August 2000 and then gunning down her mother and her mother’s friend three years later. (Mike Christy/Arizona Daily Star via AP)

TUCSON, Ariz. - The trial against a former Arizona fire captain who rose through the ranks as police suspected him in three murders concluded its first day with emotional testimony from the defendant's second ex-wife, who had provided him with an alibi.

David Watson is accused of killing his first ex-wife, Linda Watson, in August 2000 and then gunning down her mother and her mother's friend three years later.

The prosecution says both murders were tied to Watson's desire to keep full custody of his daughter with Linda Watson.

Rosemary Watson, his second ex-wife, said in court on Wednesday that she had covered for her husband on both occasions. "There was no way that I wanted to believe that it was possible that he was involved," she said through tears.

Her testimony came after opening statements in which prosecutors said sufficient evidence will show David Watson killed Linda Watson during a bitter custody battle over their then-four-year-old daughter and Marilyn Cox three years later during a long legal battle over grandparent visitation. Renee Farnsworth, a friend of Cox who frequently accompanied her to her visits with the girl, was also shot dead.

"These murders were carefully concealed, no doubt about that. These murders were well-executed. But the killer left some stuff behind. He made some mistakes," prosecutor Jonathan Mosher said.

The defence says there's no evidence linking David Watson to the killings. Attorney Michael Storie said Linda Watson was struggling with alcoholism and had just lost her job when she disappeared. He said that her off-and-on boyfriend was volatile and that Linda Watson feared him. He painted Rosemary Watson as a scorned woman after a divorce who was vengeful when she went to police in 2007 to say she had falsely provided his alibi on the nights of both murders.

"She was crushed. She was shattered," Storie said. "Rosemary goes to police in 2007 and changes her story."

Authorities said they believe that David Watson killed Linda Watson on Aug. 20, 2000, four days before a crucial court hearing in an ongoing custody battle over the couple's child.

"It was not your average fight. As nasty as those things can be, this one was worse," Mosher said.

Linda Watson's skull was found in the desert in 2003 in an area where migrants who cross from Mexico into Arizona have been found dead in the past. It would be eight years before she was identified through DNA because of a backlog at the medical examiner's office.

Cox and Farnsworth were shot dead in front of Cox's house on May 7, 2003, following an unsupervised visit she had with her granddaughter. Prosecutors say Cox was worried for her safety and would ask her friend to accompany her when she had to see David Watson.

That visit came after a two-year legal battle Cox waged seeking grandparent visitation rights. The judge ruled in her favour and forced David and Rosemary Watson to allow the girl to see her grandmother unsupervised.

Detectives from the Pima County Sheriff's Department suspected David Watson all along, but they never told the Tucson Fire Department about their concerns. Watson worked for the department for 20 years and in 2007 was promoted to captain. He resigned shortly after his arrest in April 2015.

Rosemary Watson said in court that she woke up the night Linda Watson went missing, that her husband wasn't in bed and that he was nowhere to be found. She went back to sleep, woke up around twilight and saw him cleaning out the back of his sports utility vehicle through a window, she testified.

She said that she lied to police the night Cox and Farnsworth were gunned down when she said her husband had been home since before the women dropped off the girl. Instead, she said, David Watson came home later than expected. She said he was pale, shaken up and acting strangely, asking her to wash his clothes immediately after he arrived.

"There was kind of terrifying fear factor that went with this whole thing not just of Dave but of this whole situation. I was terrified of the outcome of what would be," Rosemary Watson said. "Clearly, years and years later, we're here."

Rosemary Watson's testimony will continue on Thursday.

David Watson has pleaded not guilty to two counts of first-degree murder and one count of second-degree murder.

News from © The Associated Press, 2016