The authorities arrived at Marquez's South Pasadena apartment complex on Thursday after her landlord reached out concerning her welfare check.
Police say they spent 90 minutes at her home with a mental health professional and that she was "extremely uncooperative".
During the welfare check at the 1100 Block of Fremont Avenue, Marquez was having seizures and possibly suffering from mental health issues, Lieutenant Joe Mendoza of the Los Angeles County Sheriff's Department's Homicide Bureau, Yahoo News reports, quoting The Wrap.
Mendoza told reporters at the scene that she seemed "gravely disabled".
The weapon turned out to be a BB gun, investigators said.
She was taken to an area hospital where she was pronounced dead. Clooney said in response, "I had nothing to do with casting".
Clooney denied the allegation.
Serena Williams Kills The US Open In A Tutu After Catsuit Ban
Venus, ranked 16th, advanced to Friday's third round matchup by defeating Camila Giorgi 6-4, 7-5 earlier on Wednesday. Also into the last 32 is Elina Svitolina (7) of Ukraine after a 6-3, 6-2 win over Germany's Tatjana Maria.
In addition to her role on ER, Vanessa Marquez also had parts in Malcolm & Eddie, Seinfeld, Melrose Place, and the movie Stand and Deliver. Marquez said that George Clooney spearheaded a campaign to have her blacklisted in the entertainment industry.
Marquez had played nurse Wendy Goldman in the first several seasons of the NBC hit medical drama series "ER".
In 2005, Marquez's shopping addiction was covered in an episode of Intervention.
"I take her at her word", he told US Weekly in 2017. I was an actor and only an actor. She accused George Clooney of booting her from the show and blacklisting her. saying she was the victim of racist and sexist comments. "Women who don't play the came lose career" I did. "She is identified as a female Hispanic 49 years-old and resident of South Pasadena".
Her Twitter account has since been suspended for reasons unclear.
"A person only has so much strength and I'm afraid I've used all mine up", she wrote at the time. She also alleged that cast members Anthony Edwards, Noah Wyle and Julianna Marguiles had been racially insensitive on set. Marquez tweeted that the blacklisting began when she made complaints to the show's executive producers, John Wells.