Back from the dead? Detectives thought 13-year-old Mary Day was murdered – so who was the woman claiming to be her decades later?

Did a man who said he had a "demon" in him kill the teen, bury her in a shallow grave, and then hire an impostor to take over the murdered teen's identity for cold hard cash?

What Ever Happened to Mary Day?

Along the rugged central California coast, the legend of Mary Day haunted two generations of homicide detectives around Monterey.

Since the mid-1990s there had been rumors a young girl, just 13 years old, had vanished without a trace from her home near the massive old Army base at Fort Ord.

But there was no record that anyone had ever reported her missing. Detectives in the small military town of Seaside, California, never even got their hands on the case until more than 20 years had passed.

It was Mary's sister, Sherri Calgaro, who finally got authorities on the case. Sherri was 10 when Mary disappeared. Calgaro was always told that Mary had run away, but she never accepted that and when she grew up, she finally reported her sister missing.

Detectives began to piece together Mary's life.

Since Mary's stepfather William Houle was a soldier, life for the family had been transient — they moved from base to base around the country, so it wasn't that unusual that no one really knew the family. Still, it was strange that they never even enrolled their daughter in a local school.

Mary Day
Is the woman pictured at right the same person as the teen at left who disappeared in 1981? Sherrie Calgaro/Phoenix Police Dept.

Mary was a ghost.

Detectives could find no trace of Mary, no IDs issued, no paychecks, school records, arrest records — nothing, for over 20 years. They quickly became convinced Mary had been murdered. Cadaver dogs alerted near a tree in the backyard where the family lived at the time she disappeared, and police dug up a girl's shoe.

In an interview with police, Mary's stepfather admitted having a big fight with the girl. He didn't kill her, he said, but a demon inside of him could have.

Detectives were also suspicious of Mary's mom, Charlotte Houle. They believed she may have been complicit in covering up the crime. Detectives say she was uncooperative with their investigation and seemed not to care about finding out what happened to Mary.

When detectives asked her why she wasn't interested in finding Mary, she just answered, "If she's dead, she's dead."

Then, just as homicide detectives were building their case against Mary's stepfather, the most extraordinary thing happened. Police in Phoenix, Arizona, pulled over a pickup truck in a routine traffic stop. The woman in the truck told them she was Mary Day and she had the ID to prove it.  Police were suspicious. The ID had been issued just months after detectives interviewed Mary's parents.

Detectives in California could not believe it. Mary was their homicide victim, they believed they had a near confession from her stepfather and they were actively searching for her body.

Police ordered DNA tests, and to everyone's shock, the woman calling herself "Mary" did turn out to be Charlotte's daughter. She must be the long-lost Mary Day, right?

New insight on decades-old case of missing girl Mary Day

Perhaps not. Detectives and some family members were suspicious. The woman they began calling "Phoenix Mary" couldn't remember important facts about her childhood, she had a thick Southern accent, and she didn't know about an inheritance that Mary was due. 

When the woman from Phoenix collected Mary's inheritance, detectives wondered if they were dealing with an impostor — an impostor who shared the DNA of the "real" Mary.

There were stunning new developments before the investigation reached its conclusion. 

Correspondent Maureen Maher reports in "What Ever Happened to Mary Day?" an all-new "48 Hours" airing Saturday, May 2 at 10/9c on CBS.