The video starts with officers entering her home for a wellness check.
Marquez told officers she was having a seizure.
Police say paramedics and a mental health clinician spoke with her right after that.
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Officers told Marquez they were going to take her to a local hospital to be evaluated. They say that's when she picked up a handgun, and officers retreated downstairs.
Police say they opened fire when Marquez advanced toward them with the weapon.
Marquez was killed in the confrontation.
Investigators magnified the video to show a hand holding a gun. The weapon was later determined to be a replica.
Marquez's family has filed a wrongful death lawsuit against the city of South Pasadena.
Marquez was 49 years old. She was best known for her roles on "ER" and "Stand and Deliver."