ET has obtained a copy of Hefner's death certificate from the Los Angeles Department of Public Health, which lists the immediate cause of death as cardiac arrest.
Secondary causes include respiratory failure, septicemia and drug resistant E. Coli. The E. Coli is listed as being from an "undetermined etiology," meaning the origin of the highly dangerous bacteria could not be confirmed.
Hefner died on Sept. 27 at 5:06 p.m., according to the certificate, and his occupation was listed simply as "Playboy Founder."
A source close to Hefner's family told ET after his death, "Hef hadn't been doing well for the last year. [His death] wasn't shocking to the family."
"Everyone is obviously sad, but they feel he lived a very fulfilling and long life," the source added.
For more on the iconic publisher's life, watch the video below.