Four people are dead after a group of five reportedly jumped in unison from a seventh-floor apartment balcony around 7 a.m. Thursday in the Swiss resort town of Montreux on the banks of Lake Geneva, Switzerland.

Vaud police spokesman Alexandre Bisenz said the group seems to “have visibly thrown themselves into the void from an apartment” landing hard on the pavement below, where they were found by passersby.

Four people were pronounced dead at the scene. A survivor is said to have potentially catastrophic injuries, and is in grave condition at a local hospital.

None of the jumpers were wearing shoes, according to witnesses who saw the bodies.

Neighbors on the seventh floor of the residential apartment block told police that a family of five—two adults in their forties, two teenagers, and a grandmother—moved into the flat three years ago.

Police did not confirm whether the jumpers were that family. “We heard nothing from their home, the father never said hello in the hallway and ordered many packages almost daily,” Claude Rouiller, who lives nearby, told Le Temps newspaper. He added that a strong smell of incense had been wafting from under the apartment door for the last few days.

One neighbor who saw the bodies on the pavement told the paper that many people in the building thought the family belonged to a cult.

Many of the residents heard the sound of impact as the bodies hit the ground, reportedly at the same time, which led police to refer to the incident as a “group jump.”

Forensic police are combing the apartment for clues about what led to the horrific tragedy, including whether anyone else could have been in the apartment leading up to the incident. Residents shared photos and theories on the apartment WhatsApp group, according to local media.

“It was a quarter to seven, I got out of the shower, had a coffee and heard a thud,” a neighbor told the paper. “I thought it was a bad film, I couldn’t look at the image for more than a few seconds.”