The lead U.S. Forest Service investigator looking into the cause of Colorado's largest wildfire testified Tuesday that she doesn't believe the official version of how it started -- that a burning letter sparked the fire.Agent Kimberly Jones was testifying in a Denver federal civil case where five insurance companies and several property owners are suing the federal government for more than $7 million. The plaintiffs argue that the government acted negligently in the first few minutes of the fire.A Forest Service employee, Terry Barton, was convicted of starting the 2002 Hayman wildfire and spent nearly six years in a federal prison before being released this summer.Jones told the judge she never believed Barton's story that the fire started accidentally, after she burned a letter from her husband and put it in a fire pit.When Jones testified that she didn't believe there ever was a letter, the judge interrupted the questioning to ask Jones if she had testified to that previously. She told the judge she was never asked that question on the stand before.Jones, who interviewed Barton two days after the fire began, believes Barton staged the fire so she could put it out and then be hailed as a hero.Jones was then asked if that meant Barton lied when she stipulated everything in her plea bargain was fact. Jones replied, "Yes."The plaintiffs are arguing that Barton burned a letter in a campfire ring to make the case that the Forest Service is responsible for the 138,000-acre fire that ultimately destroyed 133 homes and 466 outbuildings over three weeks in June 2002.Barton was a Forest Service worker who had been assigned to enforce the fire ban when she said she burned a two-page letter from her husband in a fire ring near Lake George, Colo. The fire spread in the dry tinder, sparking the wildfire, Barton said.Due to extremely dry conditions the fire ran 17 miles by the next day. Barton did not admit to starting the fire until after her arrest.The civil trial began on Monday and will likely wrap up on Thursday.Federal District Judge Wiley Daniel is listening to testimony and will rule on the lawsuit from the bench.Barton was listed as a potential witness, but it now appears she won't testify.