A Southern California man has been convicted of killing his business associate and his family over money, and burying their bodies in the desert.

Jurors on Monday found 62-year-old Charles “Chase” Merritt guilty of four counts of first-degree murder in the bludgeoning deaths of Joseph McStay, McStay’s wife, Summer, and the couple’s 3- and 4-year-old sons.

Prosecutors said Merritt killed the family with a sledgehammer at a time when he owed McStay money and was being cut out of the victim’s business making and selling custom water fountains.

Merritt worked with McStay in his water features business.

The jury also found the special circumstance of multiple murders. Prosecutors have said they would seek the death penalty if Merritt was convicted, and the penalty phase of the trial was scheduled to begin Tuesday.

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The family vanished from their San Diego County home in 2010, puzzling investigators.

After the McStay family disappeared, authorities found bowls of uneaten popcorn at their San Diego County home, which had no signs of forced entry, and their car parked at a strip mall near the Mexican border.

For years, officials couldn’t determine what happened to the McStays. At one point, investigators said they believed the family had gone to Mexico voluntarily, though they couldn’t say why.

In 2013, their bodies were found in shallow graves in the desert after an off-road motorcyclist discovered skeletal remains in the area. Authorities also unearthed a rusty sledgehammer that they said was used to kill the family.

Merritt was arrested in 2014.

Authorities said they traced Merritt’s cellphone to the area of the desert gravesites in the days after the family disappeared and to a call seeking to close McStay’s online bookkeeping account.

Merritt referred to McStay in the past tense in an interview with investigators after the family vanished, and while the evidence linking him to the killings is largely circumstantial, it is “overwhelmingly convincing,” Imes said.

Merritt’s attorneys said the two men were best friends and investigators overlooked another possible suspect in the killings. Instead, they said, authorities zeroed in on an innocent man, but the evidence didn’t add up, noting there were no signs of an attack inside the family’s home.

“They tried his character and not the facts of this case,” defense attorney James McGee told jurors.

Many questions still remain about the family’s disappearance. Prosecutors acknowledge details of the killings aren’t entirely clear but say the evidence from the family’s car, cellphone towers and financial accounts link Merritt to the killings.

Authorities said McStay was cutting Merritt out of the business in early February and the two met on Feb. 4 in Rancho Cucamonga, where Merritt lived at the time.

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Prosecutors said financial records show Merritt tried to loot the business bank accounts just before and after the family disappeared and backdated checks to Feb. 4, knowing it was the last day anyone had contact with McStay.

Phone records show McStay called Merritt seven times after the Feb. 4 meeting, with defense lawyers arguing that McStay wouldn’t likely do that if he had just fired Merritt.

The Associated Press contributed to this report.