Four family members—including an 8-year-old girl and 41-year-old twin sisters—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.

Police say all of the jumpers were French nationals and part of the same family living in the apartment. In the early hours of Thursday morning, authorities had tried to follow up on a wellness check related to one of the children, who was being homeschooled, but police were unable to enter the home and left.

A short time later, the family appears to have jumped off the balcony.

According to a statement from police, two officers knocked at the door “to execute an arrest warrant issued by the prefecture in connection with the home schooling of a child. The mandate concerned the father, a 40-year-old Frenchman residing in this apartment.”

“The gendarmes knocked on the door and heard a voice asking them who was there,” the statement continues. “After having announced themselves, the gendarmes then heard no more noise in the apartment. Unable to contact the potential occupants, they left the premises. In the meantime, a witness called the police to report that people had fallen from the balcony of an apartment.”

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: an 8-year-old girl, the child’s mother and father, and the mom’s twin sister. A 15-year-old boy 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.

Denis Balibouse/Reuters

Neighbors on the seventh floor of the residential apartment block told police that a family of five—a husband and wife in their 40s, two children, and a grandmother—moved into the flat three years ago. The wife’s twin sister was not registered as a resident there.

“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.”

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