A Southern California man was convicted Monday of killing a family of four and burying their bodies in the desert in a case that puzzled investigators for years after the family suddenly vanished from their home in 2010. After a trial that spanned more than four months and depended largely on circumstantial evidence, jurors in San Bernardino found 62-year-old Charles "Chase" Merritt guilty in the bludgeoning deaths of his business associate Joseph McStay, McStay's wife, Summer, and the couple's 3- and 4-year-old sons.

Merritt closed his eyes and looked down when the court clerk said the word "guilty" the first of four times to first-degree murder. Sobs came from the packed courtroom. Someone called out, "Yes!"

Charles "Chase" Merritt, second left, stands in a San Bernardino courtroom after a jury finds him guilty of four counts of first-degree murder in the 2013 killings of the McStay family. AP

Prosecutors say 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 of making and selling custom water fountains.

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The jury also found special circumstance of multiple murders, which means Merritt is eligible for the death penalty. 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.

Prosecutors declined to comment after the verdict and families on both sides left without speaking to reporters.

After the McStay family disappeared, authorities found bowls of uneaten popcorn at their San Diego County home, which showed no signs of forced entry. Their car was later found 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.

"It was blow, after blow, after blow to a child's skull," the Los Angeles Times reported prosecutor Britt Imes said during closing arguments.

Merritt, who worked with McStay in his water features business, was arrested in 2014. Merritt had been working on a book about the McStay family murders when he was taken into custody, CBS News reported at the time.

In the book, titled "Afraid of the Light," Merrick claimed Joseph McStay had been concerned his wife may have been poisoning him. He was looking to release it later this year. Patrick McStay, Joseph McStay's father, told CBS News in 2014 that Merritt had talked to him about the book in a phone conversation.

"He wanted me to read what he was writing. He proceeded to start reading it to me. He said, 'Why don't we collaborate together and write a book?'" Patrick McStay told CBS News.

Summer McStay, left, Joseph McStay, right, and their two children, Gianni and Joseph Jr. Family photo

Patrick McStay said it immediately made him suspicious of Merritt, but he "played along" in hopes of finding out what happened to his son, his daughter-in-law and his two grandchildren.

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 acknowledged that details 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.

Prosecutors say 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.