The mystery of the family's 2010 disappearance was largely treated as a missing-persons case until their bodies were discovered in 2013

By Chris Harris
November 20, 2017 05:45 PM

In 2010, a picture-perfect family of four vanished without a trace four months after moving into a new house in the San Diego suburb of Fallbrook, California.

Three years later, in November 2013, a motorcyclist stumbled upon a skull in the Mojave Desert and called investigators who — soon after — uncovered two shallow graves containing the remains of a man, a woman and two little boys.

The missing McStay family had been found.

The mystery of who killed them and why will be the focus of Monday night’s episode of People Magazine Investigates on Investigation Discovery.

The disappearance of Joseph McStay, 40, his 43-year-old wife, Summer, and sons Gianni, 4, and Joseph Jr., 3, was largely treated as a missing-persons case until their bodies were discovered in 2013.

As authorities learned, the foursome, who were last seen alive on Feb. 4, 2010, had all been viciously beaten to death.

You can watch an exclusive preview of Monday’s episode above. The clip alludes to a possible motive in the slayings: money, and lots of it.

In 2014, authorities filed murder charges against Charles “Chase” Merritt, Joseph’s onetime business partner. Merritt, who maintains his innocence, will stand trial for the killings this coming January.

According to investigators, DNA allegedly matching Merritt was found on the steering wheel of the McStays’ white Isuzu Trooper, which was abandoned near the Mexico border following the family’s disappearance.

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The McStay family

The episode features interviews with Joseph’s father, Patrick McStay, as well as PEOPLE Senior Editor Alicia Dennis and Elaine Aradillas, a PEOPLE staff writer.

According to detectives, Joseph’s body was found with a white extension cord wrapped around his neck. Summer’s skull was fractured and one of their sons’ heads was bludgeoned seven times, according to court testimony.

A sledgehammer was also found buried at the site.

Merritt has pleaded not guilty to the four counts of murder he faces.

His attorney, who could not be reached for comment on Monday, has previously told reporters there is no evidence that links his client to the slayings.

People Magazine Investigates airs Monday (10 p.m. ET) on Investigation Discovery.