Los Angeles firefighters responded to a medical call about 8 a.m. at the 8,000-square-foot home Murphy shared with her husband, British screenwriter Simon Monjack, in the hills above the Sunset Strip.
There were conflicting reports as to whether Murphy was discovered by Monjack or her mother.
Firefighters tried unsuccessfully to revive Murphy on the way to Cedars-Sinai Medical Center, where she was pronounced dead about two hours later.
Los Angeles police are investigating the cause of Murphy's death, said Officer Norma Eisenman. Investigators and coroner's officials were at her home Sunday as paparazzi camped outside.
Authorities said Murphy was taking prescription drugs for a variety of medical ailments and said preliminary indications do not point to foul play.
An autopsy is planned for Tuesday, a coroner's official said.
Reached at his home in northern Florida, Murphy's father, Angelo Bertolotti, said he learned of her death Sunday morning from his son (Murphy's half brother), who read it on the Internet.
"It's too shocking and too fast," Bertolotti said in a phone interview. "It just doesn't make sense to me. As we speak I am staring at a picture of her. I just can't believe it. Can't convince myself she's gone."
Bertolotti, who described himself as a "retired mobster" who now works in the movie business, said he and Sharon Murphy divorced when Brittany was 4. He last saw his daughter about three years ago.
Bertolotti said his daughter was very close to her mother.
"They were inseparable," he said.
A statement from Murphy's family released late Sunday by her publicist, Nicole Perna, called the actress' death "a terrible tragedy."
"She was our daughter, our wife, our love and a shining star," the statement said. "We ask you to respect our privacy at this time."
Born Nov. 10, 1977, in Atlanta, Murphy moved to Burbank with her mother when she was 13.
After rising to prominence in the 1995 film "Clueless," Murphy appeared in the Winona Ryder-Angelina Jolie drama "Girl, Interrupted," and the Michael Douglas thriller "Don't Say a Word."
A series of lighter roles followed the 2002 drama "8 Mile," including a lead role opposite Ashton Kutcher in the 2003 romantic comedy "Just Married."
Murphy also lent her voice to animated characters, including Luanne Platter on the Fox TV show "King of the Hill" and Gloria the penguin in the 2006 feature "Happy Feet."
Director Curtis Hanson, who worked with Murphy on "8 Mile," said the actress' personal attributes combined easily with her on-screen work.
"She was a very special and vulnerable person," Hanson said. "The honesty of her emotions made it so that [her lines] didn't feel like scripted dialogue."
Murphy's Hollywood life sometimes crossed paths with her personal life. In 2004 she broke off an engagement with Jeff Kwatinetz, a principal at the company that managed her career.
Murphy also briefly dated Kutcher.
On Sunday, he sent a Twitter message saying, "2day the world lost a little piece of sunshine. My deepest condolences go out 2 Brittany's family, her husband & her amazing mother Sharon."
Murphy, who most recently starred in "Across the Hall," an indie feature in limited release, is expected to be seen in the upcoming Sylvester Stallone action movie "The Expendables."
There have been reports that Murphy left production before work was complete on her current project "The Caller." That account was parodied on a recent "Saturday Night Live" episode.
Funeral arrangements were pending.
ching-ching.ni@latimes.com
andrew.blankstein @latimes.com
Times staff writer Steve Zeitchik contributed to this story.
