Thai police say they are seeking two US citizens over the suspected theft of body parts - including a baby's head and an adult's heart - from a hospital.
The men were questioned on Saturday after staff at a DHL depot in Bangkok discovered three parcels containing the human remains, bound for the US.
The men were freed after saying they had bought the items at a market to give to friends as a joke.
The Americans are now believed to have travelled to neighbouring Cambodia.
Police said that staff at a DHL depot in Bangkok were doing a routine check on several parcels labelled as toys when they made the discovery.
Deputy national police chief Ruangsak Jaritake told reporters the authorities had received information indicating that the body parts had been "stolen from one of the big hospitals" in Bangkok's Thonburi neighbourhood.
"It is a famous hospital," he said. "But we are still looking for clearer evidence."
Police named one of the Americans as a controversial video maker, Ryan McPherson.
The BBC's Jonathan Head in Bangkok says he became notorious for making violent exploitative videos a decade ago about homeless men, and was questioned by police then.
'No grounds to prosecute'
Police in Bangkok say the parcels were bound for addresses in Las Vegas. One reportedly had a baby's head, a baby's foot sliced into three parts, an adult heart with a stab wound, and pieces of adult human skin with tattoos.
The police said they were also investigating a possible murder because of the stab marks.
The items were in five plastic containers filled with preserving fluid, packed into three packages. DHL staff discovered them when putting the boxes through an X-ray machine.
Police said the body parts appeared to have been removed and preserved by a medical professional.
The containers looked similar to those used to preserve corpses at Bangkok's Siriraj Medical Museum, according to AFP news agency.
One of the two men who tried to post the items was questioned by Thai police for four hours on Saturday night in the presence of US embassy officials, before he was released, the Nation newspaper reports.
Police told reporters they had released the men as they had no grounds to prosecute them. The FBI in the US has been informed about the case.
In 2012 a Briton was arrested in Bangkok for possession of six foetuses wrapped in gold leaf. Police had been tipped off that the foetuses were being sold through a website advertising black magic.