PENSACOLA, Florida (CNN) -- Authorities have located valuable evidence, including a stolen safe and several guns, in the slaying of a Florida couple who had adopted 13 children, a state attorney said Thursday.
Byrd and Melanie Billings did not have any children together, but they cared for 16 children over the years.
Bill Eddins said the safe had been taken from the couple's home. He said authorities found several guns, including one they believe was used to kill Byrd and Melanie Billings.
The two were shot to death July 9 in their Beulah, Florida, home with nine of their children present.
"In our opinion this was a home invasion-robbery where the people stole a safe and we recovered the safe," Eddins told reporters at a news conference. "I personally ... think it is as simple as that in terms of the motive."
Sheriff David Morgan of Escambia County, Florida, said authorities do not believe that anything else is missing from the Billingses' home.
Earlier, police said they planned to interview two more people in the killing of the Billingses, whose adopted children included many with special needs.
Those interviews may provide a more complete picture of the circumstances surrounding the slayings, Morgan said on CNN's "American Morning."
"We believe -- but this case has had so many odd twists and turns -- that this may be the last piece of our puzzle," he said.
Investigators have already charged eight people in the case. One of the two they plan to interview "possibly could have a connection to the case," Morgan said. He referred to them as "persons of interest" but did not elaborate.
The sheriff compared the investigation's complexity and the number of people involved to the infamous 1969 killings in California for which Charles Manson and others were convicted.
Seven men -- one of them a 16-year-old -- are charged with murder, and one woman faces accessory to felony murder charges. Watch expressions of relief after arrests »
Police detained the eighth suspect Wednesday night, Pamela Long Wiggins, Morgan said.
Wiggins was previously identified as Pamela Laverne Long.
Morgan told CNN on Thursday that investigators had obtained physical evidence from her.
He said Wiggins rented property to the alleged ringleader of the crime, Leonard Patrick Gonzalez Jr., and was the suspect's family friend.
Law enforcement officials earlier said a safe and other items were missing from the house and have cited robbery as one motive, but said there may be others. Watch sheriff describe crimes »
The crime lasted only minutes after seven men entered the Billingses' home, Morgan said earlier.
He said authorities were looking into what he called a "gaping hole" in what was otherwise a well-executed crime -- why the security system was not disabled. Authorities believe the suspects did not believe they were under surveillance, he said.
The surveillance video showed two vehicles pulling up to the home and five people dressed in black and wearing masks entering it. Watch surveillance video of the home invasion »
Authorities believed the drivers remained in both vehicles.
Morgan said earlier that authorities are seeking a person who was involved in the security and video surveillance system at the Billingses' compound.
The Billingses had 13 adopted children -- three of those children have died over the years. Byrd and Melanie Billings also each had two biological children of their own but no biological children together.
Nine of the couple's children were home during the killings, authorities said, and police believe three of them saw the intruders. One of the children managed to flee and seek help at a neighbor's house, Morgan has said.
Gonzalez Jr., 35, a former National Guard member, appeared in court Wednesday and read a statement proclaiming his innocence.
"The state's entire case is based on hearsay and circumstantial evidence," he said via video.
His father, Leonard Patrick Gonzalez Sr., 56, also appeared in court Wednesday. He is being held without bail, as are Gary Lamont Sumner, 30, and Fredrick Lee Thornton, 19.
They are to appear in court again next month.
Wayne Thomas Coldiron, 41; Donald Ray Stallworth, 28; and an unidentified 16-year-old are also charged in the case.