Investigators say she took her own life by inhaling helium.
The body of Teresa Mastracola, 44, of Millbourne, Pa., was found on Saturday.
At the time, police said they did not suspect foul play.
Then, on Monday, police said her death had been officially ruled a suicide, saying she died from helium inhalation and chemical asphyxia.
The search for Mastracola began last Sunday.
She was last seen leaving her home between 9:00 a.m. and 10:00 a.m. wearing scrubs and getting into her car.
At first, Millbourne police were reluctant to call the woman's disappearance suspicious because she is an avid hiker who often left town for days at a time.
Mastracola worked as an Imaging Equipment Specialist at Delaware County Memorial Hospital. Her coworkers started to get nervous this week when she did not show up for work.
Action News has also learned that the woman holds the rank of Master Sergeant with the National Guard.