Police in the northern canton of Schaffhausen bordering Germany said the so-called "Sarco" capsule had been deployed in a wood in the municipality of Merishausen on Monday.
Prosecutors in Schaffhausen have opened criminal proceedings against several people for "inducing and aiding and abetting suicide", a police statement said on Tuesday, adding several people were detained, without giving details about them or the deceased.
A spokesperson for the group behind the capsule, The Last Resort, said the deceased was a 64-year-old American woman who had been suffering from a severely compromised immune system.
Florian Willet, co-president of The Last Resort, was among the four detainees, along with a Dutch journalist and two Swiss people, the spokesperson said. Willet was the only other person present when the woman ended her life, the spokesperson said.
In a statement issued by The Last Resort, Willet had described the death as "peaceful, fast and dignified".
The Last Resort spokesperson said the woman had passed psychiatric evaluations prior to ending her life.
A spokesperson for prosecutors in Schaffhausen declined to give details or confirm there were four detainees.
Cast along sleek, aerodynamic lines, the "Sarco" causes death when its occupant releases nitrogen gas inside, lowering the amount of oxygen to lethal levels. It is the brainchild of Philip Nitschke, an Australian physician famous for his work on assisted suicide since the 1990s.
Switzerland has been a magnet for advocates of assisted suicide due to laws that make it legal there, and The Last Resort says its legal advice was that it could be deployed.
The capsule has generated considerable media attention and discussion among authorities on whether they would allow it.
Elisabeth Baume-Schneider, the Swiss minister responsible for health, said on Monday that the capsule does not meet the requirements of product safety law, and that its use of nitrogen is not legally compliant.Â
