Finnish police said Monday that the man arrested for the murder of a local politician and two journalists at the weekend was suffering from a mental disorder.

Flags were flying at half mast in the small lakeside town of Imatra, which was in shock after the killing of the three women in Saturday's late-night shooting.

"On current information, police consider it likely that the cause of this deed was linked to social exclusion and mental health (problems)," the police department in southeastern Finland said in a statement.

Police said on Sunday that the 23-year-old suspect acted alone and there were no indications that the triple murder was linked to "political issues or extremism".

The suspect, who had previous convictions for violence, shot the women at close range as they were leaving a restaurant in Imatra, hitting them in the head or torso.

The gunmen was quickly arrested without any resistance and interrogated on Sunday but refused to give a reason for his murderous spree, police said.

"This is a shared grief even if everyone did not personally know the victims. This affects us all," Imatra council official Arja Kujala was quoted as saying by FNB, the Swedish-language news agency in Finland.

Finland enjoys relatively low crime rates compared to other European nations but, with many Finns keen hunters, it has one of the biggest gun ownership rates in the world.

The Nordic nation has seen several deadly shootings over the past decade, all by young men.

In a makeshift memorial to the three slain women, candles were laid out in the snow outside the Vuoksenvahti restaurant where the attack took place.