Paris Hilton is mourning the loss of her "longtime friend" and fellow heiress, Casey Johnson.

"I feel so sad right now, my heart is in pain," Paris writes on her Twitter page after learning of Casey's death. "Just lost a longtime friend. My heart goes out to her family. R.I.P. Casey Johnson Love you :("

The Johnson family has been friends with the Hilton family since before Paris and Casey were born. Paris' sister Nicky is godmother to Casey's daughter Ava, 3.

Hilton continues, "In bed crying, looking at baby pictures of Casey, Nicky and I. I feel so upset. I feel like I've lost a sister. My heart is broken. Miss her."

Johnson was originally asked by Hilton to co-star in the reality series, "The Simple Life." Johnson turned it down, so the role went to Paris' other pal, Nicole Richie. In 2006, Casey told Vanity Fair that her decision to pass on the role was "the stupidest mistake of my life". She added, "I kick myself in the butt every day."

Paris concludes her Twitter posts with, "God has another Angel by his side. Casey, we all Love and Miss you and know that you are in a better place. R.I.P."

Everyone at ParisHiltonJustMe are deeply saddened by your the loss of Casey and our condolences her family, friends and loved ones. Rip.