Four women and a man were killed and two others wounded when a man used a bow and arrow to attack them in Norway.
Police first received word of an attack in the town of Kongsberg, south-west of the capital Oslo, at 18:12 local time (16:12 GMT).
A Danish man aged 37 has been arrested and questioned for hours overnight.
Police said they had previously been in contact with him over fears of radicalisation after he converted to Islam.
The victims were all aged between 50 and 70, regional police chief Ole Bredrup Saeverud told reporters on Thursday morning.
He said they were most likely killed after the police first confronted the attacker at 18:18.
Reports of the incident were "horrifying", said Prime Minister Erna Solberg, hours before she was due to leave office.
"I understand that many people are afraid, but it's important to emphasise that the police are now in control," she said.
The attacker is said to have launched the assault inside a Coop Extra supermarket on Kongsberg's west side. One of those injured was an off-duty police officer who was in the shop at the time.
A spokesperson for the chain later confirmed a "serious incident" at their store, adding that none of their staff were physically injured.
Local police chief Oyvind Aas confirmed that the attacker had managed to escape an initial confrontation with police before an arrest was finally made at 18:47 local time, 35 minutes after the attack began.
One witness told local outlet TV2 she had heard a commotion and seen a woman taking cover, then a "man standing on the corner with arrows in a quiver on his shoulder and a bow in his hand".
"Afterwards, I saw people running for their lives. One of them was a woman holding a child by the hand," she added.
Police have told Norwegian news agency NTB that the attacker also used other weapons during the incident, without giving more details on what they were.
The suspect moved over a large area, and authorities cordoned off several parts of the town. Residents were ordered to stay indoors so authorities could examine the scene and gather evidence. Surrounding gardens and garages were searched with the help of sniffer dogs.
The attack was Norway's deadliest since far-right extremist Anders Behring Breivik murdered 77 people, most of them at a children's Labour Party summer camp on the island of Utoya in July 2011.
Kongsberg Mayor Kari Anne Sand said it was a shocking attack that had taken place in an area where many people lived, and that a crisis team would help anyone affected.
Describing the town as "a completely ordinary community with completely ordinary people", Ms Sand said everyone had been deeply shaken by "this very tragic situation."
The suspect was taken to a police station in the town of Drammen, where his defence lawyer, Fredrik Neumann, said he was questioned for more than three hours and was co-operating with authorities.
The suspect had a Danish mother and Norwegian father, he explained.
Norway's outgoing justice minister Monica Maeland told reporters the police did not yet know whether or not it was act of terrorism and could not comment on details emerging about the suspect.
Police prosecutor Ann Irén Svane Mathiassen told TV2 that the man had lived in Kongsberg for several years and was known to police.
The attack came on the final day of Erna Solberg's conservative government, and a new justice minister takes over the case on Thursday under a centre-left coalition led by Labour leader Jonas Gahr Store.
Mr Store said it was a "gruesome and brutal act", hours before announcing his new cabinet.
Norwegian police are not usually armed and after the attack the police directorate ordered all officers nationwide to carry firearms as an extra precaution.
Police were searching the Huseby area of north-western Oslo on Thursday following reports of a man being seen carrying a bow and arrow. Police stressed no-one had been hurt and there was no threat.
"The police have no indication so far that there is a change in the national threat level," the directorate said in a statement (in Norwegian).