(CNN) -- Sweden's capital endured a fourth consecutive night of rioting Wednesday, with fires and clashes with authorities decreasing in the original flashpoint in the northern suburbs but increasing to the south, police said.
Rioters continued to set fires to vehicles and other structures in Stockholm and suburbs, and they pelted responding firefighters and police with rocks and other objects, Stockholm police press officer Kjell Lindgren said Thursday.
Early media reports said the four days of riots, which started in the high-unemployment, low-income northern suburb of Husby on Sunday, might have been triggered by last week's shooting death of a man by police. But police said Wednesday they were not sure of the cause.
Among Wednesday's incidents: Rioters set fire to a restaurant and tried to burn two police stations, Lindgren said. The fires at the police stations were extinguished before the stations were destroyed.
"In the southern parts of our city ... we've seen an increase in violence, and most of our police officers and firemen who were working overnight have said that this has been very difficult, with so many incidents in so many places," Lindgren said.
He said 10 fires were set in the northern Stockholm area overnight, but he didn't have a number for the southern suburbs. The Local, an English-language online newspaper, reported that Stockholm's fire service had responded to at least 75 incidents throughout the city by 2 a.m. Thursday.
Police were so busy escorting firefighters, "we've only had time to arrest one person" overnight -- a 16-year-old girl on suspicion of preparing arson, Lindgren said. Eight people were arrested in Husby on Tuesday night, according to police.
More than 100 vehicles were set on fire Sunday night just in Husby, police said. Another 29 were set on fire Tuesday night in the wider district, they said.
Husby is an area that has a lot of problems and a high crime rate, according to Lindgren.
Tensions have been brewing since May 13, when police shot dead a 69-year-old Husby man who had a machete, The Local newspaper said.
On Wednesday, Stockholm police spokesman Lars Bystrom told CNN that authorities weren't sure why the riots began.
"We have ... heard the reports that some are making the connection to the 69-year-old man who was shot dead by police this past Monday on May 13 in his apartment after he threatened police with a machete, and the officers attending felt their life was in danger and had to react," Bystrom said. "But we don't know for sure what is actually behind this. At this point, it is pure speculation.
"It would be very unfortunate if we started hesitating to go in when serious incidents happen because of this."
The Local also quoted a local youth leader as saying some police officers used racial slurs against residents Sunday as the trouble flared. Bystrom said Wednesday that police are investigating "possible use of inappropriate language or excessive use of force."
The disorder led Swedish Prime Minister Fredrik Reinfeldt to issue a statement Tuesday appealing for calm.
"We have had two nights of great worry, damage to property and a threatening mood in Husby. There is a risk that it will continue. Now everyone must help out to calm things down," he said.
"We have groups of young men who believe one should and can change our society through violence. We cannot let violence govern."
Reinfeldt said the trouble reflects a broader problem in Swedish society: More should be done to support education and help young people into work, he said.
CNN's Jason Hanna, Stephanie Halasz and Laura Smith-Spark contributed to this report.