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Tucson, Ariz. • While David Watson was rising through the ranks at the Tucson Fire Department, authorities working to solve three cold-case murders had him in mind as their main suspect.

Watson, 46, was finally arrested on Saturday, nearly 15 years after the first of the killings.

The fire department captain is accused of killing his ex-wife, her mother and a third woman, and he now faces three counts of first-degree murder.

Linda Watson and her ex-husband were involved in a bitter custody battle when she was reported missing on Aug. 21, 2000.

Her body was found in the Tucson Mountains in west Tucson in 2003 among the remains of immigrants who have died while walking the treacherous desert about 70 miles north of the Mexican border, sheriff's office spokesman Tracy Suitt said.

But her body remained unidentified at the medical examiner's office for several years before DNA testing helped identify her in 2011, Suit said.

Watson is also accused of killing Linda Watson's mother, 63-year-old Marilyn Cox, and Cox's friend, 53-year-old Renee Farnsworth, on May 7, 2003. The women were gunned down at the home of Linda Watson after Cox returned from a court-mandated visit with her granddaughter, Suitt said.

Cox had been in a legal battle with Watson over the girl. The girl was Linda Watson and David Watson's only child.

Suitt said he couldn't delve into details about what led to Watson's arrest this weekend, saying it could hurt the case.

"We are dealing with a person who is very dangerous, and if we give out some information it will jeopardize this case. We're gonna hold off on some of the details," Suitt said.

Watson has worked for the Tucson Fire Department for 20 years and in 2007 was promoted to captain, but the fire department was not notified he was a suspect in the murders, Suitt said.

"We can't just call them up and say this person is a suspect in homicide until we have enough evidence. That would be irresponsible," he said.

Watson is on unpaid leave, fire department spokesman Barrett Baker said in a statement.

"The Tucson Fire Department is saddened to learn about the circumstances related to one of our employees. Our thoughts and prayers go out to the friends and family of the victims," he said.