BERLIN (AP) — Authorities in Germany say four people have died after being treated at a hospital on the Dutch border that offers alternative cancer treatment.

Police in Moenchengladbach announced the first death, of a Dutch woman who had received treatment for breast cancer at the Biologisches Krebszentrum in Brueggen-Bracht, on Monday. On Thursday, they said another three patients who were treated last week have died in recent days — one from Belgium and two from the Netherlands.

Police said in a statement that the cause of the deaths isn't clear but they are calling on anyone who received treatment at the center to contact authorities.

The hospital said Wednesday it is giving full cooperation to the police investigation.