The "Sarco" suicide capsule, which has never been used before, is designed to allow a person inside to push a button that injects nitrogen gas into the sealed chamber.
The person is then supposed to fall asleep and die of suffocation in a few minutes.
Prosecutors in Schaffhausen canton were informed by a law firm that an assisted suicide involving the use of the Sarco capsule had taken place Monday near a forest cabin in Merishausen, police said in a statement on Tuesday.
It said "several people" were taken into custody and prosecutors opened an investigation on suspicion of incitement and accessory to suicide.
Dutch newspaper Volkskrant reported Tuesday that police had detained one of its photographers who wanted to take pictures of the use of the Sarco.
Dr Phillip Nitschke says lawyers advised him using the capsule would be legal in Switzerland. (Joe Castro/AAP PHOTOS)
It said Schaffhausen police indicated the photographer was being held at a police station but declined to elaborate.
The newspaper declined to comment further.
Dr Philip Nitschke, the Australian-born trained doctor behind Exit International, told Associated Press his organisation received advice from lawyers in Switzerland that the use of the Sarco would be legal in the country.
Exit International, an assisted suicide group based in the Netherlands, has said it is behind the 3D-printed device that cost more than $A1.5 million to develop.
Swiss law allows assisted suicide so long as the person takes his or her life with no "external assistance" and those who help the person die do not do so for "any self-serving motive", according to a government website.
In July, Swiss newspaper Blick reported that Peter Sticher, a state prosecutor in Schaffhausen, wrote to Exit International's lawyers saying any operator of the suicide capsule could face criminal proceedings if it was used there - and any conviction could bring up to five years in prison.
Prosecutors in other Swiss regions have also indicated that the use of the suicide capsule could lead to prosecution.
A 54-year-old US woman with multiple health ailments had planned to be the first person to use the device earlier this year, but those plans were abandoned.
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