Advocacy groups behind a so-called suicide capsule said Sunday they have suspended the process of taking applications to use it — which numbered over 370 last month — as a criminal investigation into its first use in Switzerland is completed.

The president of Switzerland-based The Last Resort, Florian Willet, is being held in pretrial detention, said the group and Exit International, an affiliate founded in Australia over a quarter century ago.

Swiss police arrested Willet and several other people following the death of an unidentified 64-year-old woman from the U.S. Midwest who on Sept. 23 became the first person to use the device, known as the “Sarco,” in a forest in the northern Schaffhausen region near the German border.

Others initially detained were released from custody, authorities have said.

Switzerland has some of the most permissive laws in the world when it comes to assisted suicide, though the first use of the Sarco has prompted a debate among lawmakers.

Laws in the rich Alpine country permit 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.”

The advocacy groups said in a statement Sunday that 371 people were “in the process of applying” to use the Sarco in Switzerland as of Sept. 23 and applications were suspended after its first use.

Exit International, whose founder Dr. Philip Nitschke is based in the Netherlands, is behind the 3D-printed device that cost over $1 million to develop.

The Sarco capsule is designed to allow a person sitting in a reclining seat inside to push a button that injects nitrogen gas from a tank underneath into the sealed chamber, allowing the person to fall asleep and then die by suffocation in a few minutes.

Exit International has said Willet was the only person present at the woman’s death, and described it as “peaceful, fast and dignified.” Those claims could not be independently verified.

On the same day as the woman died, Swiss Health Minister Elisabeth Baume-Schneider told parliament that use of the Sarco would not be legal. The woman was said to be suffering from severe immune compromise.

Exit says its lawyers in Switzerland believe use of the device is legal.

“Only after the Sarco was used was it learned that Ms. Baume-Schneider had addressed the issue,” the advocacy groups said in the statement Sunday. “The timing was a pure coincidence and not our intention.”

(AP)