It could have been that the person was hit directly or was hit by parts of the vehicles during the collision, police spokesperson Fredrik Svedemyr says.
The injured woman was flown by ambulance helicopter to hospital with serious injuries, according to local media. A man and two children were also taken to hospital. The man had minor injuries, but there is no indication that the children were injured.
One person has been arrested on suspicion of gross negligence in traffic and causing grievous bodily harm.
According to witness accounts, the car with the caravan was parked along the road when the collision occurred.
One lane on the E6 is still closed but is expected to open at 7 p.m., according to the Swedish Transport Administration.
