Police in Ohio have arrested a family of four who are suspected of killing eight members of another family on their cannabis farm in 2016.
The murder of the Rhoden family was "meticulously planned" and was related to a child custody dispute, police say.
The Wagner family, who have been arrested, moved to Alaska after the crime and later returned to Ohio.
Ohio's attorney general said the suspects "spent months planning the crime" but made mistakes along the way.
"The killers knew the territory and meticulously planned these murders," Attorney General Mike DeWine said.
George "Billy" Wagner III, 47, and Angela Wagner, 48, and their two sons, 27-year-old George Wagner IV and 26-year-old Edward "Jake" Wagner were arrested on Tuesday in connection the 2016 killings.
They face more than 80 criminal charges, including eight counts of aggravated murder which could land them on death row.
It's unclear if the Wagners have a lawyer, but they have previously denied committing the murder several times over the course of the two-year investigation.
In addition, Mrs Wagner's 65-year-old mother, Rita Newcomb, and Billy Wagner's 76-year-old mother, Fredericka Wagner, have also been charged with obstruction of justice and perjury. They are not believed to have taken part in the killings, according to police.
Who are the victims?
On 22 April 2016, eight members of the Rhoden family were shot to death in four separate homes on their family farm in Pike County, Ohio - about 100 miles (150km) from Cincinnati - "under cover of darkness", Mr DeWine said.
They were identified as Hannah Gilley, 20; Christopher Rhoden, Sr, 40; Christopher Rhoden, Jr, 16; Clarence "Frankie" Rhoden, 20; Dana Rhoden, 37; Gary Rhoden, 38; Hanna Rhoden, 19, and Kenneth Rhoden, 44.
The victims were found in their beds, and appeared to have been asleep, according to investigators.
A four-day old infant, a six-month old baby, and a three-year-old child were all spared during the attack.
Jake Wagner was the long-time boyfriend of victim Hanna Rhoden, and shared custody of their daughter Sophia.
Sophia, now five years old, was not present at the family home at the time of the murders, and is in child protective custody, according to police.
"There certainly was obsession with custody, obsession with control of children," Ohio Attorney General Mike DeWine said. He declined to provide further details regarding their alleged motive.
"This is just the most bizarre story I've ever seen," he said, adding that it "has been by far the longest, most complex and labour intensive investigation the Ohio Attorney General's Office has ever undertaken".
Jake Wagner is also accused of sexual contact with a minor in connection to his relationship with Hanna Rhoden, who was 15 at the time. He was 20 when they first started dating.
What have officials said?
Mr DeWine, who was elected last week as Ohio's next governor, said the Wagners used their familiarity with the Rhodens to plot their murders.
"They knew what they were doing. They thought about it. A lot," Mr DeWine said at a news conference on Tuesday.
"They were brutally and viciously executed," he continued. "The killers knew the territory and meticulously planned these horrendous murders."
During the investigation, Angela Wagner told local media that Hannah Rhoden "was like a daughter" to her and called the killers "monsters".
Her husband, Billy Wagner, was also said to be close friends with victim Kenneth Rhoden.
Pike County Sheriff Charles Reader, who was nearly brought to tears during the news conference, told reporters: "Members of one family conspired, planned, carried out and then allegedly covered up their violent act to wipe out members of another family."
"They did this quickly, coldly, calmly and very carefully - but not carefully enough.
"They left traces, they left a trail - the parts to build a silencer, the forged documents, the cameras, cell phones, all that they tampered with. And the lies, all the lies they told us."
During the investigation, about 100 cannabis plants were found on the victims' property, and officials had initially speculated that the murders could be drug-related.
Mr DeWine said the drugs were unrelated to the motive, but that the probe did include "an undercurrent of drugs".