The agency, known by its acronym PST, said Thursday that Wednesday night's attack in a small Norwegian town “currently appear to be an act of terrorism.”
“The investigation will clarify in more detail what the incidents were motivated by," it said in a statement.
It added that the suspect “is known to PST from before, without PST being able to provide further details about him.”
Police said Thursday that the Danish man suspected of the attack is a Muslim convert who was previously flagged as having been radicalized.
“Attacks on random people in public places are a recurring modus operandi among extremist Islamists carrying out terror in the West,” the domestic security agency said.
The agency said that “the most probable scenario of an extremely Islamist terrorist attack in Norway is an attack carried out by one or a few perpetrators with simple weapon types, against targets with few or no security measures.”
PST says the terror threat level for Norway remains unchanged and was considered “moderate.”