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.
Family of 'ER' actress killed by police in South Pasadena in 2018 demand answers
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."