“There is no possible natural explanation for this type of fire to break out” by itself, Gustaf Sandell told public radio.
The blaze comes days after an arsonist set fire to a mosque in central Sweden on Christmas Day, injuring five people.
Sweden, known for tolerance and refugee-friendly policies, saw an extreme right party, the Sweden Democrats, become the third-largest force in parliament in legislative elections in September.
In early December, the party brought down the government after less than three months by refusing to back its budget proposal in parliament.
On Saturday the government announced it had reached a deal with the opposition that will enable it to remain in power and to avert the Nordic country’s first snap elections in more than half a century, but the far right has threatened to hold a no-confidence vote.