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SAN DIEGO — A teenager started a fire in her backyard last May that sparked the larger Cocos wildfire, which blackened nearly 2,000 acres and destroyed three dozen homes, a prosecutor told a judge Tuesday.

Deputy District Attorney Shawnalyse Ochoa made her remarks as a non-jury trial got under way for the minor, who is charged with five arson counts. The trial before Judge Howard Shore is expected to last two weeks

Ochoa told the judge that the accused — who was 13 at the time — went into her backyard with a lighter last May 13 and set a branch on fire. The teenager then went back in the house and told her sister, “There’s a fire out back,” according to the prosecutor.

Firefighters responded and extinguished the small blaze, but the next day, the girl posted a photo of a fire burning in Carlsbad on social media, then went out to her backyard again, where she set another fire, this one spreading to nearby trees, Ochoa said.

Two CAL Fire investigators determined that an ember from the fire behind the girl’s home traveled .44 of a mile to spark the Cocos fire, according to the prosecutor.

Ochoa said the teenager intentionally, willfully and maliciously set both fires behind her home, and urged the judge to find her guilty.

Defense attorney Ryan McGlinn said his expert would dispute the prosecution theory that an ember traveled almost a half-mile to ignite the Cocos blaze.

“We’re talking a phantom ember,” McGlinn told the judge.

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