Monjack, 40, was pronounced dead after the Los Angeles Fire Department was called to the residence for a medical emergency, say authorities. Police spokesman Sgt. Louie Lozano said the preliminary cause of death was natural causes, but investigators remained at the scene.
Law enforcement sources told the Hollywood Reporter that Brittany's mother, Sharon, discovered Monjack around 9:30, unresponsive, and called 911.
In January, Monjack's mother, Linda Monjack, told PEOPLE that her son was "unwell, and the doctors are carrying out tests. On whether he has a heart problem it is not really for me to say, you must ask him, but yes, there have been health problems in the past. I believe it's common knowledge, and it’s been in the press that he had a slight heart attack a week from Brittany's death coming back from Puerto Rico."
Murphy, 32, died in December due to pneumonia, a low-blood count and multiple drug intoxication. The couple, who married in 2007, had no children.
After Murphy's death, a Los Angeles coroner official, Assistant Chief Ed Winter, told PEOPLE: "This death could've been preventable. Murphy was planning on seeing a doctor, but she unfortunately passed away before she did. This was a case of a person with pneumonia who was anemic [having a low blood count] who was taking medication when she should've been getting medical treatment."
In an interview with PEOPLE in January, Monjack maintained that his wife had visited a doctor. "I took very good care of my wife ... She was on an antibiotic and she was taking cough medicine and doing all the right things," he said.
Regarding concern from Murphy's friends that Monjack should be investigated, Winter said: "He was never considered a suspect. No one's criminally liable here because this has been ruled an accident."
Additional reporting by MONIQUE JESSEN