Monroe's housekeeper, Eunice Murray, noticed that the light in
Monroe's room was on late at night, but that Monroe did not respond
to Murray's knocking. Murray called Monroe's psychaitrist Ralph
Greenson, who arrived within minutes. Greenson looked through a
window and saw Monroe lying on her bed, so he broke a window to get
inside. The doctor found her dead (or nearly so), so he called the
police.
The EXACT timeline is still in dispute.