The encounter near the Angeles National Forest in the 2200 block of Canyon Road occurred shortly after someone reported the big cat in their back yard about 7:50 p.m., Arcadia police Lt. Larry Goodman said.
An Australian shepherd that belonged to the residents was barking at the cougar, then suddenly fell quiet, he said.
When the officers entered the backyard, they saw the mountain lion standing over the dog. The cat then charged at the officers, prompting them to open fire, he said.
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The carcasses of both animals were turned over to the Pasadena Humane Society, pending retrieval by the Department of Fish and Game, which will investigate the shooting.