German police have arrested a 51-year-old woman on suspicion of killing four people at a hospital in Potsdam.
Authorities were called to the Oberlin clinic in the eastern German city shortly before 21:00 CET on Wednesday.
Four people were found dead, and a fifth person seriously injured, in patient wards at the facility, Brandenburg state police said.
"The injuries of all victims were caused by severe, external violence," police added in a statement.
The female suspect detained had been "urgently" sought in the case and was an employee at the clinic, they said. No information has been given for a possible motive for the killings.
Forensic experts and the public prosecutor's office are still at the scene near the German capital city, Berlin.
An investigation into the killings is underway, led by Brandenburg police and the Potsdam Public Prosecutor's Office.
The victims were residents of Thusnelda-von-Saldern-Hau clinic, which cares for people with serious physical or mental disabilities.
Police say they are informing the relatives of those who were killed, but have not released any more details.
The mayor of Potsdam, Mike Schubert has described the incident as an "incomprehensible act".
"Our thoughts today are with the relatives of those killed, the residents and all employees," he tweeted, "we will be at the side of the in this difficult time."
The theological head of the facility - which also has a hospital, schools and workplaces for people with disabilities - has also expressed shock in a statement.
"We are stunned because we cannot understand this act in any way," said Dr Matthias Fichtmüller in a statement.
"All of our concern and sympathy goes to the relatives of those affected and to our colleagues in Thusnelda-von-Saldern-Haus. We pray fervently that our resident in the hospital will survive their injuries."