Florian Willet, a euthanasia advocate who was detained by Swiss authorities last year after being present when an American woman ended her life using a chamber-like device, has died.

Mr. Willet’s death was reported in an obituary posted on the website of The Last Resort, his assisted dying group, written by Philip Nitschke, the inventor of the device, known as a Sarco capsule. Mr. Nitschke said in an email that Mr. Willet had died by assisted suicide, but further details about his death remained unclear. The police in Germany, where Mr. Willet died, could not immediately be reached for comment.

Mr. Willet, who was 47, according to the obituary, was the only person with the American woman when she died using the Sarco device in a remote forest in Switzerland in September. He was arrested, along with three others, by the Swiss authorities, who said at the time that the group was under investigation for “aiding and abetting suicide.”

The incident amplified thorny questions surrounding assisted dying even in Switzerland, where laws around the practice have led thousands of people to seek assisted death from right-to-die organizations based there in recent years.

Mr. Willet was released from pretrial detention in December, after which “he was a changed man,” Mr. Nitschke wrote.

“Gone was his warm smile and self-confidence. In its place was a man who was deeply traumatized by the experience of incarceration and the wrongful accusation of strangulation.”