One of the wounded was an off-duty police officer who had been in a store, one of the several places attacked.
"The man who committed this act has been arrested by the police and, according to our information, there is only one person involved," Mr Aas told the news conference.
He said that "given how events unfolded, it is natural to assess whether this is a terrorist attack".
"The arrested man has not been interviewed and it is too early to say anything about his motives," he said, adding that "all possibilities were open".
Several people killed in bow and arrow attack in Norway
Norway's intelligence service PST had been alerted, spokesman Martin Bernsen told AFP.
"It is all conjecture at the moment," he said when asked about the motive.
Police said the suspect had been taken to a police station in the nearby town of Drammen but gave no other details about the man, including whether he had previously been known to the authorities.
Police were informed of the attack at 6.13pm (3.13am AEDT) in the town of 25,000 people, around 80km west of the capital Oslo. The suspect was arrested at 6.47pm local time.
"These events shake us," said Prime Minister Erna Solberg on her very last day in office.
On Thursday she will hand over the prime ministership to Jonas Gahr Store, whose Labour Party won parliamentary elections on 13 September.
Arrow sticking in the wall
The scene of the attack was blocked off by a police, an AFP correspondent said.
Police urged the public to stay at home and several neighbourhoods were cordoned off, with television footage showing ambulances and armed police in the area.
Police in the Scandinavian country are not normally armed, but after the attack the National Police Directorate ordered that officers be armed nationwide.
A helicopter and bomb disposal team were also sent to the scene.
The website of public broadcaster NRK published an image sent by a witness of a black arrow sticking out of a wall.
In other pictures from the scene, what looked like competition-grade arrows could be seen lying on the ground.
Norway has traditionally been a peaceful nation, but it suffered one of history's worst mass shootings in 22 July 2011 when right-wing extremist Anders Behring Breivik killed 77 people.
Breivik first set off a bomb in Oslo next to the building that housed the office of the prime minister, then went on a shooting spree at a summer camp for left-wing youths on the island of Utoya.
In another right-wing attack, self-proclaimed neo-Nazi Philip Manshaus opened fire into a mosque on the outskirts of Oslo in August 2019 before being overpowered by worshippers, with no one being seriously injured.
However, he had earlier shot dead his step-sister, who had been adopted from China, in what prosecutors termed a "racist act".
Several planned jihadist attacks have also been foiled by security services.