The shooting occurred in an apartment in Stora Höga in Stenungsund and police were called to the scene at 12.15pm.
According to a police spokesperson, a car occupied by one of those involved in the shooting was witnessed leaving the scene.
Police have classified the shooting as extraordinary and have called in units from across the area to assist in the search.
“We cross jurisdictions and are using all units, we are pooling all the resources that we have. We consider the incident to be very serious,” police spokesperson Stefan Gustafsson to news agency TT.
Police remain restrictive with regard to releasing details around the shooting, as well as the ongoing hunt for the perpetrators. No suspects had been apprehended by 2pm on Monday.
“We are searching several locations for the car which has been mentioned,” Gustafsson said.
Any potential motives for the shooting remain unclear.