Charles "Chase" Merritt is accused in the 2010 deaths of Joseph and Summer McStay and their two young boys.
Merritt has pleaded not guilty. He was a former business partner of Joseph McStay, who sold outdoor water displays over the Internet.
The newly released documents show that investigators spoke with Dan Kavanaugh, another partner in the business. He told them Joseph McStay had loaned Merritt $30,000 to pay a gambling debt, and that McStay had planned to fire Merritt.
Investigators with the San Diego County Sheriff's Office also spoke to Merritt on several occasions, the documents show.
He told them he did not like Summer McStay, nor Kavanaugh, and said: "If I were ever going to commit murder, it would be him," meaning Kavanaugh.
Merritt had a fresh injury on his hand, which he claimed he cut on sheet metal.
And although Merritt denied he'd ever driven the family's Isuzu Trooper, his DNA was found on the steering wheel and gear shift. The truck was found abandoned near the Mexican border soon after the family disappeared.
Their skeletal remains were discovered in November 2013.
Joseph, Summer and their sons Gianni, 4, and Joseph Jr., 3, were bludgeoned to death with a sledgehammer and buried in shallow graves in the high desert outside Victorville, northeast of Los Angeles.
The documents show that Merritt had a sister who lived only a few miles from where the McStays were found.
A judge determined last month
there was sufficient evidence to try Merritt.
Before his arrest, he spoke to the media about his relationship with the McStays and professed his innocence.
"I am definitely the last person (Joseph McStay) saw," Merritt told CNN's Randi Kaye.