PENSACOLA, Florida (CNN) -- Investigators are making "very delicate moves" in probing the killings of a Florida couple known for adopting special-needs children, the local sheriff said Friday.
Byrd and Melanie Billings, known for adopting special-needs children, were laid to rest Friday.
Seven people have been arrested on murder charges in the July 9 killings of Byrd and Melanie Billings, whose funeral was held Friday. An eighth person faces charges as an accessory after the fact.
"We are now at a phase in this investigation that is much like master's-level chess," Escambia County Sheriff David Morgan told reporters. "We are making very delicate moves. We are very concerned about any misstep that we may take or that we may make that would jeopardize the successful prosecution of the individuals we currently have in our custody. This is a difficult, difficult thing to do, so please bear with us." Watch sheriff say more will be revealed »
The Beulah, Florida, couple was shot to death in a home-invasion robbery while nine of their children were in the house. A safe taken from their home contained only prescription medication, family documents and some jewelry, Morgan and a family lawyer said.
The safe was found in the backyard of Pamela Long Wiggins, a friend and landlord of one of the suspects. It was buried in the ground with bricks piled atop it in "an obvious attempt to conceal the safe," Morgan said. Watch Spencer reveal the safe's contents »
The involvement in the investigation of several federal agencies -- including the FBI, the Drug Enforcement Administration and the Bureau of Alcohol, Tobacco, Firearms and Explosives -- fueled speculation that robbery may not have been the sole motive. But Morgan has played down the agencies' role in the county's investigation, and Crystal Spencer, a lawyer for the Billings family, said she disclosed the contents of the safe to put the speculation to rest.
Morgan said the contents of the safe were less significant than that it was taken and where it was found. Those facts were "critical in the interview and interrogation process to link people to elements of this crime," he said.
The 47-year-old Wiggins, described by authorities as a landlord and friend to Leonard Patrick Gonzalez Jr., one of the suspects, has been freed on a $10,000 bond. Repeated attempts by CNN to reach her have been unsuccessful.
Authorities have described Gonzalez, who proclaimed his innocence in court earlier this week, as the organizer of the crime.
Although a state attorney said authorities believe the investigation is mostly wrapped up, Morgan said earlier Friday that another arrest could come next week. He gave no further details, except to say the person was not on the Billingses' property at the time of the killings.
Surveillance video from the home captured two vehicles pulling up to the property and five people, masked and dressed in black, entering the house. Authorities believe both drivers remained in the vehicles. Watch authorities detail findings »
Morgan said earlier this week that it appears the suspects did not believe they were under surveillance, and he has pointed to the fact that the system was not disabled as a "gaping hole" in a well-executed crime.
Melanie Billings' brother, Dr. Ed Brock, told reporters that the Billingses' lives "centered around children, their family and each other." Watch latest developments »
"They gave these children a joyous childhood and a much-needed voice," he said. "... Their lives were cut way too short, but their legacy will live on."
The couple had adopted 13 children, and had two biological children each, but three of the adopted children have died. Nine of the children were at home at the time of the killings, and one managed to flee and seek help at a neighbor's house.
The funeral service for the couple on Friday drew hundreds of mourners to Pensacola's Liberty Church.
At a graveside service later at Pensacola Memorial Cemetery, church pastor Buford Lipscomb read from a letter that one of the Billingses' daughters, Missy, had written, referring to her parents and her own newborn son.
"We will tell him how the world wept with the injustice that took you from us," the letter said.
Several of the couple's younger children released balloons with messages attached.
"Words do not express the pain they feel in their hearts or great loss of this incredible couple," Lipscomb said of the family. "They loved their children beyond anything else."
CNN's Aaron Cooper, Susan Candiotti, Mike Phelan, Ed Lavandera and John Couwels contributed to this report.