Six people were in the house when the fire broke out on Friday night, but at least three managed to get out, according to local police. The other three are feared dead.
The house was completely destroyed in the fire.
On Saturday morning, local police had still not managed to search the premises and they had not been able to get in touch with the people living in the house.
Peter Adlersson, spokesperson for the police in the Västra Götaland County on the west coast of Sweden, said a number of people were taken in for questioning overnight.
He added that those questioned included witnesses but said he could not confirm whether they also included residents in the burnt-down house.
The scene of the suspected arson has been under police watch overnight and remained so on Saturday morning as the fire brigade continued its work to extinguish the fire and forensic investigators searched the scene fore clues.
Orust local government councillor, Hans Pernervik, told the TT news agency that he arrived at the scene at 8pm on Friday night. He described the fire as “completely out of control” and said locals feel shaken by the event.
“It was all burning like a torch. I didn’t know a house could burn like that,” another witness, a neighbour identified as Göran, told TT.
On Saturday morning smoke was still billowing out of the remains of the house. Hans Blohm, deputy chief of rescue operations on Orust, said he expects the fire to be fully extinguished by late Saturday morning.
According to the SOS Alarm centre an injured person who was outside the house when the fire broke out has been brought to a nearby hospital.
Eight people are registered at the address, but according to the police, who have spoken to the house owner, five of them do not live there.