It is a family's nightmare: You hire someone to help care for your elderly parent and then investigators say that person ends up stealing hundreds of thousands of dollars.

"Our father died alone, and he has six children," said Susan Lewis.

Lewis is talking about her 91-year-old father, Edward Whitehead. His final years were difficult. Dementia was robbing him of his memory and an attendant was allegedly draining his bank account.

"She portrayed herself as a caregiver. She immediately took over and controlled everything," Lewis said about Diane Harvell.

Harvell was hired to provide transportation for Lewis'  father and help with small tasks. What happened next is a warning inspectors have for all families.

"She managed to weasel her way into the family, and ostracized him from his children, managed to convince him that she was the only one he could trust," explained U.S. Postal Inspector Michael Van de Putte.

"She wanted to keep us away. If she could keep us away that way we couldn't force her into court or get more information on her," added Lewis' brother, Edward Whitehead.

Investigators said Harvell convinced the elder Whitehead to sign over his power of attorney. She then altered his will and forged notary public signatures -- all without his family knowing.

"She signed all documents as if she were his daughter. She signed (a) do not resuscitate (order)," said Lewis.

When the 91-year-old died, his family learned the truth.

"This woman was 10 steps ahead of us; the moment we found out our father was deceased… We found out that evening, the next morning she was down there filing a forged will. The next morning. Wow. That was fast," said Lewis.

Postal inspectors began poring over bank records and surveillance video and verified Harvell stole hundreds of thousands of dollars.

"She took his money in structured withdrawals, less than $10,000 from the bank, drained over $200,000 of his personal savings," said Van de Putte.

Investigators said her control and his dementia were a terrible combination.

"Senior citizens generally have savings, or a decent amount of money, and are generally easy targets because they are very trusting," Van de Putte explained.

Some advice from postal inspectors: Don't wait until it's too late to set up a plan that protects your elderly loved ones and pay attention.

"Be wary of any new person coming into your life, especially if you are dealing with a senior citizen, or a parent or grandparent that suffers from dementia (or) Alzheimer's," said Van de Putte.

"What she has done. The woman has no feelings. There is something mentally, seriously mentally wrong with this woman. She absolutely has no heart whatsoever," said Lewis.

Harvell faces mail theft, ID theft, mail fraud, forgery and several other charges. Postal inspectors said she could face up to 15 years in prison.

In Florida, to find out more about elder abuse, including how to report it and resources that are available, you can visit Florida's Department of Elder Affairs. You can also call the Florida Abuse Hotline at 1-800-96-ABUSE (1-800-962-2873). Press 1 to report suspected abuse, neglect or exploitation of the elderly or a vulnerable adult. This toll free number is available 24/7.

In Georgia, to find out more about elder abuse, reporting it and resources available,  you can go online to the Division of Aging Services. You can also call Adult Protective Services at 1-866-55AGING (1-866-552-4464). Press 3 to report abuse.  Reports are accepted by phone Monday through Friday 8 a.m. to 5 p.m.