South Pasadena police officers opened fire while they were performing a welfare check on the former ER star on Thursday afternoon.
The tragedy unfolded at around 8pm BST (12pm local time), when members of South Pasadena’s police force responded to a home in the 1100 block of Fremont Avenue.
The actress’ landlord called the authorities because he was concerned for her wellbeing.
For the following 90 minutes, three officers and a mental health care worker tried to administrate aid to the actress who was allegedly displaying “extremely uncooperative” behaviour.
Police opened fire when, at 10pm BST (2pm local time), Ms Marquez pointed in the officers’ direction a weapon they said looked like a semiautomatic handgun.
The actress was struck at least once in the torso, local media The South Pasadenan reported.
She was rushed to hospital, but she died for the injuries reported.
Sheriff’s Homicide Lt Joe Mendoza said police realised Ms Marquez’s gun was just a replica when it was seized from her house.
The actress was suffering from eating disorders and appeared to be gravely disabled, Mr Mendoza added.
Police said they had been to the actress’ home on many occasions in the past, KABC reported.
The Sheriff’s Homicide Bureau has launched an investigation into the officer-involved shooting.
Ms Marquez reached fame in the early 1990s, when she played nurse Wendy Goldman on the popular NBC medical drama ER starring, among others, George Clooney between 1994 and 1997.
But her relationship with Hollywood star Mr Clooney deteriorated when, last year, she accused the actor of blacklisting her after she spoke out about alleged harassment on the set of ER.
On October 9 2017 Ms Marquez tweeted: “Clooney helped blacklist me when I spoke up about harassment on ER.
“‘Women who don’t play the game lose career.’ I did.”
The actress said she was subjected to racist and sexist remarks on set.
Mr Clooney, who spoke out against Harvey Weinstein as a string of sexual harassment allegations swept over the businessman, replied saying he had no idea Ms Marquez had been blacklisted and he didn’t play any part in it.
He said in a statement: “I had nothing to do with casting. I was an actor and only an actor.
“If she was told I was involved in any decision about her career then she was lied to.
“The fact that I couldn’t affect her career is only surpassed by the fact that I wouldn’t.”