WARNING: GRAPHIC CONTENT
WHEN FBI investigators entered a Detroit warehouse in 2013, they were met with a grisly sight.
Human remains were found "frozen together in flesh-on-flesh chunks" in the space with no running water or heating.
The "cutting room" floor was covered in blood, while the place was strewn with piles of dead flies, gallon drums and blunt instruments like a chainsaw and a circular saw.
That's what FBI agent Leslie Larson told a jury this week at the trial of businessman Arthur Rathburn, a man accused of supplying customers body parts infected with HIV and hepatitis.
"Body parts were out in the open, in coolers," Ms Larson told the jury. "Some of the freezers had heads and torsos, some had arms and legs.
The jury heard how the body parts were frozen in clumps so that a crowbar was needed to separate them. Mr Rathburn has denied the charges against him and his lawyer, James Howarth, blamed another person.
"This case is so sensitive because the nature of the evidence is going to make us all cringe, make us all uneasy," he said.
"There's nothing particularly pretty about a deceased body that has been separated into parts, but I would hope no one would have bad feelings toward Mr Rathburn because of that."
The grisly case has shone a spotlight on a chilling and shadowy industry in the US that few realise exists - the trade in body parts for research and education.
While strict laws govern the use of organs like hearts and livers for transplants, there is a large and unregulated industry where body parts can be bought and sold for medical research, often without consent or awareness from the families involved.
Separate to the court case, a recent Reuters investigation showed how reporter Brian Grow was able to purchase two human heads and a cervical spine from a Tennessee broker over email for less than $US1000 ($A1275).
He found the spine came from 24-year-old Cody Saunders who died of a heart attack after having 66 surgeries and 1700 rounds of dialysis in his short life. His family had donated his body to science because his parents "couldn't afford nothin' else".
The example highlighted how the broker business can prey on poor families, sometimes offering free cremation in exchange for the body parts.
From there, bodies can be dismembered and sent to buyers around the country that range from medical centres to research units, although the Reuters sale found no verification checks were done on the buyer.
Typical prices include $US750 ($A950) for a brain, $US300 ($A380) for a spine, $US200 ($A250) for a foot or $US2000 ($A2500) for a torso.
The organisation found some companies were making millions in annual revenue from the trade while one had an efficiency model based on McDonald's. Brokers range from national companies to small family outfits with varying levels of professionalism, ethics and hygiene.
In the case of one company, families were allegedly offered free cremation in exchange for helping "advance medical studies".
In reality, neighbours of the site complained of finding bloody boxes in the rubbish. When health inspectors visited they found a "man in medical scrubs holding a garden hose," Reutersreports.
"He was thawing a frozen human torso in the midday sun."
Another case revealed the story of a man who donated the brain of his 74-year-old dementia-suffering mother to science, only to find later it had been used by the US Army in a bomb blast experiment.
University of Minnesota Medical School body donation expert Angela McArthur described the current situation as a "free for all" and likened it to grave robbers of the 18th century.
"I don't know if I can state this strongly enough," Ms McArthur said. "What they are doing is profiting from the sale of humans."
The industry is complicated by the fact doctors say donated bodies are essential for training and research, as a way of trialling complex new procedures and developing new treatments.
The lack of regulation at present means while many view the industry as unethical and unseemly, it's often difficult to pinpoint exactly what laws have been broken.
Yeshiva University anatomy and structural biology professor Todd Olson said the great risk is that "nobody is watching".
"We regulate heads of lettuce in this country more than we regulate heads of bodies," he said.