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.