A “nasty” custody battle led a former Tucson firefighter to kill three women and bury one of them in the desert, a Pima County prosecutor said Wednesday.

The defense attorney for David Watson, who worked in the Tucson Fire Department from 1995 to 2015 and faces three murder charges, said authorities misread the evidence and failed to tie Watson, 47, to the slayings. Instead of Watson, they should have paid more attention to a former boyfriend of one of the women.

The remarks came during the first day of Watson’s trial in Pima County Superior Court, where he was charged in April 2015 with two counts of first-degree murder and one count of second-degree murder.

Watson is accused of killing his ex-wife, Linda, 35, in 2000. After she disappeared from her house in the 2600 block of West Curtis Road, sheriff’s deputies found a broken coffee cup and blood on a cooler, vacuum cleaner, and the floor.

The couple had been involved in a custody battle over their daughter, age 4 at the time, Deputy County Attorney Jonathan Mosher told the jury Wednesday. “As nasty as those things can be, this one was worse,” Mosher said.

Fearing he would lose custody of his daughter, the then-firefighter killed Linda Watson and buried her body near Silverbell Mine, Mosher said.

Watson also is accused of shooting and killing Linda’s mother Marilyn Cox, 63, and Cox’s neighbor Renee Farnsworth, 53, three years later.

Cox had accused Watson of her daughter’s disappearance, which “to put it simply, was a big deal in this town,” Mosher said as a photo of a billboard asking for information about the disappearance was displayed on the wall behind him.

“Marilyn was going to get to the bottom of what happened to her daughter,” Mosher told jurors.

A judge allowed Cox visitation rights to Watson’s daughter, Mosher said. On the night of the first unsupervised visitation, in May 2003, Cox and Farnsworth drove back to Cox’s house after dropping the daughter off at David Watson’s house.

When they got out of the car, a lone gunman stepped out of the oleander shrubs that lined the driveway and shot them both, Mosher said.

“This was not a robbery; this was an assassination,” Mosher said, noting the two women’s purses and phones were left untouched.

The gun used in the 2003 shooting was never found, but Mosher told the jury Watson owned a handgun that matched a model that could have been used in the shooting. Watson told detectives he had sold the gun.

In both Linda Watson’s disappearance and the 2003 shooting, David Watson’s then-wife, Rosemary, provided him with an alibi, Mosher said.

But after years of carrying the burden of lying on her husband’s behalf and a few months after their divorce, Rosemary Watson contacted detectives in 2007 — the same year Watson was promoted to captain — and said she wanted to change her story, Mosher said.

She told detectives David Watson left their house in the middle of the night that Linda disappeared in 2000. He returned hours later and she saw him cleaning his truck. Watson told her he had gone for a walk to clear his head, Mosher said.

Rosemary Watson told detectives that on the night of the 2003 shooting, David Watson returned home around the time of the shooting “sweating, pale, eyes wide, and panicked,” Mosher said. He kissed their daughters goodnight, told Rosemary to wash his clothes, and then took a shower.

Several weeks later, Cox’s relative found a gold-colored money clip in the yard of the house. The clip was inscribed with the initials “D.D.W.,” which Mosher said corresponded to “David Dwayne Watson.”

Hunters found Linda Watson’s skull in a desert area in 2003. Eight years later, the remains were identified as hers through DNA testing.

The delay in identifying her remains was due to authorities believing the skull may have belonged to a border crosser who died in the desert west of Tucson and whose remains were sent to the county medical examiner for identification, Mosher said.

Defense attorney Michael Storie accused authorities of focusing on Watson when the evidence was pointing them in a different direction.

Storie said the divorce between David and Linda Watson was not contentious and the couple agreed in court to joint custody of their daughter.

Linda was struggling with a drinking problem and her then-boyfriend asked David and Rosemary Watson to take custody of the daughter, Storie said. An emergency custody hearing was scheduled for four days after Linda Watson disappeared, he said.

Linda Watson and her boyfriend, who Storie described as a jealous man with a temper, were constantly fighting. The boyfriend urged David and Rosemary Watson to take custody of Linda and David’s daughter.

Linda changed the locks on the house and called a lawyer about an order of protection because she was so afraid of her boyfriend, Storie said, adding Marilyn Cox also had an order of protection against him.

Linda told friends she wanted to run away after she lost her job that week, Storie said.

“That’s what’s going on leading up to the (Aug.) 20th, the last time we see Linda,” Storie said.

When Linda disappeared, Marilyn Cox called the boyfriend and accused him of kidnapping her daughter, Storie said.

Storie cast doubt on how a relative could find the money clip in a gravel yard weeks after the night of the 2003 shooting when sheriff’s personnel had scoured the yard for evidence.

With regard to Rosemary’s account, Storie said David Watson “crushed her” in 2006 when he said he didn’t want to be married to her anymore and he was seeing another woman. They got divorced and soon after she goes to the police and changes her story, he said.

“She was never afraid of David. Never,” he said.

Contact Curt Prendergast at 573-4224 or cprendergast@tucson.com. On Twitter @CurtTucsonStar.