The head of an association that helped an American woman to die in a suicide capsule is being detained in Switzerland amid an inquiry into her death and reports that “strangulation” marks were found on her neck.

Florian Willet, president of the Last Resort, the right-to-die group that provided the capsule, has been in jail for more than a month after the unnamed 64-year-old woman became the first person to die in one by pressing a button that filled it with nitrogen.

Prosecutors want to determine whether he and other people involved had “selfish” motives. The woman had skull base osteomyelitis, a rare bone infection that a source close to the Last Resort suggested may have caused the neck marks.

It is legal to assist a suicide in Switzerland unless there are “selfish reasons” such as financial gain or revenge. The Last Resort, which is funded by donations, says it charged nothing for the use of its capsule except the cost of the nitrogen, 18 Swiss francs (£16). It says there are no grounds for suspecting self-serving motives.

Willet, who is German but lives in Switzerland, said beforehand that he thought he had a 10 per cent risk of being placed in detention, according to an account of the woman’s last hours in De Volkskrant, a Dutch newspaper — one of whose photographers was present when she got into the capsule.

The outlet said it had seen film taken by cameras, one inside the capsule, the other outside.

The woman is said to have pressed the button to fill the pod with nitrogen at 3.54pm and taken a deep breath. “Keep on breathing,” Willet, 47, said from outside the capsule. She lost consciousness after about 30 seconds.

The internal camera provided intermittent images, switching on in response to movement. It came on after 1 minute and 57 seconds and 2 minutes and 13 seconds, according to De Volskrant, which reported that Willet had told police these images coincided with body cramps.

Willet, who was standing beside the woman throughout the event, was heard by the newspaper telling Philip Nitschke, the pod designer, over a video call: “She’s still alive, Philip”. That was six and a half minutes after the user pressed the button to end her life.

Willet is said to have been confused by the sound of an alarm, thought to be a heart-rate monitor. The court heard how he continued to lean over the Sarco pod and peer inside until the alarm ceased.

He is reported to have said afterwards: “She really looks dead”.

Neue Zürcher Zeitung, a Swiss newspaper, and De Volkskrant said that an autopsy had discovered “strangulation” marks on the woman’s neck, prompting Peter Sticher, the public prosecutor in Schaffhausen, to extend the scope of the investigation to include the possibility of murder.

However, Sticher has refused to confirm the reports and De Volkskrant says the film taken by the external camera does not show the capsule being opened before the woman’s death.

The Last Resort and Exit International, the right-to-die association founded by Nitschke, described the allegation as “ridiculous and absurd” in a joint statement. Neue Zürcher Zeitung said “a person close” to the Last Resort said the woman’s skull base osteomyelitis may have caused the marks on her neck.