A policeman, his two children and his estranged wife were killed in a shooting at a home in Dordrecht.
Dutch police say the policeman, named as Wendell C, was the suspected gunman. It was believed to be a “family drama”.
The 27-year-old mother died in hospital on Monday night. The shooting happened in the city’s Heimerstein district.
The children – reported to be Wendell C’s daughters – were aged eight and 12. The policeman was aged 35. The city is about 20km (16 miles) from Rotterdam.
Local newspaper BN DeStem reported that the couple were separating but still lived together.
A police statement said there were “strong indications that this was a family drama”.
A city councillor, Alexander Jansen, lives in Heimerstein and told Dutch news website AD he was shocked because “this is a very quiet neighbourhood – nothing really happens here”.
“I don’t know of any safety issues – I’d like to know what was behind this.”
Dordrecht has a population of some 120,000.