A 64-year-old woman took her own life on September 23 inside the space-age looking Sarco capsule at a Swiss woodland retreat, outside a village near the German border.
Several people were arrested at the scene, with all but one being quickly released.
The public prosecutor in the northern canton of Schaffhausen did not name the remaining suspect in custody.
However, The Last Resort, an assisted dying organisation, had recently said the association's co-president Florian Willet -- the only other person present at the death -- was the man still being held.
The public prosecutor's office said in a statement that it had originally opened criminal proceedings on the grounds of incitement and aiding and abetting suicide, with strong suspicion of intentional homicide.
"Based on the latest investigation status, there is still a strong suspicion of the crime of incitement and aiding and abetting suicide, but no longer of intentional homicide, even if the autopsy report... is not yet available," it said.
"The public prosecutor's office has therefore released the last detained person from custody," it said, adding: "The presumption of innocence applies."
Pod fills with nitrogen
The Last Resort presented the Sarco pod in Zurich in July, saying they expected it to be used for the first time within months.
The capsule fills with nitrogen and causes loss of consciousness and death by hypoxia within five minutes, according to the organisation.
The portable human-sized pod, self-operated by a button inside, has raised a host of legal and ethical questions in Switzerland. Active euthanasia is banned in the country but assisted dying has been legal for decades.
Swiss law generally allows assisted suicide if the person commits the lethal act themselves, and The Last Resort said it saw no legal obstacle to its use in the country.
However, on the same day the Sarco was used, Switzerland's Interior Minister Elisabeth Baume-Schneider told lawmakers that the device was "not legal".
The Last Resort said the person who died -- who was not named -- was a 64-year-old woman from the midwestern United States.
She "had been suffering for many years from a number of serious problems associated with severe immune compromise", the organisation said.
The Sarco was invented by Philip Nitschke, a leading global figure in right-to-die activism.
The 3D-printable capsule cost more than 650,000 euros ($680,000) to research and develop in the Netherlands over 12 years.
The organisation said future reusable Sarco pods could cost around 15,000 euros.
