According to Region Östergötland, it involves a man and a woman in their 40s, as well as two small children, a boy and a girl.
The conditions of the woman and the girl are life-threatening. Both are admitted to the intensive care unit.
The man and the boy are said to be lightly injured, but will remain for observation.
According to Region Östergötland, a fifth person was also taken to the hospital in connection with the accident. The person is said to be lightly injured and was able to be sent home. It is unclear how the person sustained their injuries.
The alarm about the accident came in just before 6 pm on Tuesday.
The person who witnessed the accident immediately began trying to assist the four people in the car when they ended up in the water, says Mats Pettersson, press spokesperson for the police.
By 8 pm, the car is said to have been recovered from the scene, according to the rescue service. The police have, among other things, heard witnesses on the scene.
It is too early to say why the accident occurred.
We have a fairly clear picture of how we think it went, but the car has been seized to rule out technical fault. At present, we have initiated an investigation into bodily harm, but we'll see how it looks when we get a clearer picture of what happened, says Erik Creutz, the police's duty officer in Region Öst.
