An employee at a clinic for disabled people outside Berlin has been arrested on suspicion of killing four people at the centre and wounding a fifth, police said, with the motive still unclear.
The victims, two women and two men, were stabbed last night at the facility in the city of Potsdam, the daily Bild reported.
Those killed are believed to be patients at the clinic, local newspaper Potsdamer Neueste Nachrichten reported.
Police said earlier the victims were subjected to "intense, extreme violence", but did not give any details around the circumstances of the killings.
A 51-year-old female staff member has been arrested on "strong suspicion" of carrying out the assault, police said, while acknowledging that any motive was still undetermined.
State police were called to the clinic at around 9pm yesterday, according to reports, with the victims later discovered in their rooms.
Specialised teams were dispatched overnight to collect evidence from the crime scene.
The clinic, run by the Lutheran Church's social welfare service, specialises in helping those with physical and mental disabilities and offers live-in care as well as schools and workshops.
Around 65 people live at the facility, which employs more than 80 people.
Potsdam is the capital of Brandenburg state, whose premier Dietmar Woidke said he was "shocked by this horrible news".
"My thoughts are with the victims and their loved ones," he said, calling it a "difficult day" for the region.
Potsdam mayor Mike Schubert called the crime an "incomprehensible act".
Local residents left flowers, cards and candles at the clinic, as police maintained a large presence outside.
Germany has seen a number of high-profile murder cases from care facilities.
In the most prominent trial, nurse Niels Hoegel was sentenced in 2019 to life in prison for murdering 85 patients in his care.
Hoegel, believed to be Germany's most prolific serial killer, murdered patients with lethal injections between 2000 and 2005, before he was eventually caught in the act.
And in October, a Polish healthcare worker was sentenced to life in prison in Munich for killing at least three people with insulin.