Authorities have kept quiet for months about what drove them to charge a California man with killing a family of four and burying their bodies in shallow desert graves.

San Bernardino prosecutors on Monday are expected to finally reveal their case against 58-year-old Charles "Chase" Merritt for the murders of his business partner, Joseph McStay, the man's wife and their two young sons.

The McStay family vanished in 2010. In 2013, the family's remains were found 100 miles from their Fallbrook home.

Merritt has pleaded not guilty. After a preliminary hearing, a judge will decide whether there is enough evidence for a trial.

Merritt's lawyer, Jimmy Mettias, says he expects prosecutors will allege his client killed the family with a sledgehammer after a business dispute. Prosecutors declined to comment.