Four people died on Monday in a fire at a retirement home in Bedburg-Hau in the western German state of North Rhine Westphalia.
Police said the fire broke out during the night.
The investigation is focusing on a 71-year-old resident who is suspected of having caused the fire through negligence, the police and public prosecutor's office said.
"Against the background of the ongoing investigation, no further details can be given about the offence," the investigators added. The presumption of innocence applies.
Police said the people who died were two 50- and 74-year-old women and two 66-year-old men who had lived in the residence.
More than 20 people were injured.
"According to what we know so far, the fire broke out in one room of the residence and then spread to other rooms," said the police and public prosecutor's office.
Reports said that 46 residents of the retirement home had to be evacuated. The exact cause of the fire was still unclear.
When the fire brigade arrived at around 5 am (0400 GMT), the situation was dramatic, WDR reported, citing a fire brigade spokesman. Some of the firefighters had to break windows to rescue the residents.