“We've noticed that after this incident things are in motion. There's talk about taking matters into their own hands. As the security service we have to monitor this and make sure we can intervene before something happens,” Anders Thornberg, head of security police Säpo, told public broadcaster SVT.
“We're noticing that there are a lot of people who are upset. There's talk of revenge,” he added.
Säpo has previously listed the threat of terrorism by far-right groups as 'low' but has allocated resources to keep an eye on white power movements in Sweden, whose activity is believed will increase ahead of the parliament election in autumn next year. However, violent Islamism is still believed to be the main threat, according to Säpo, in particular so-called 'lone wolf attacks' carried out by a single individual.
Terror expert Magnus Ranstorp told SVT that he believes revenge attacks are unlikely.
“Retaliation does not happen very often. It has to be on their (the terrorists') conditions you convince someone to carry out that kind of attack. You might perhaps not be able to do that immediately after an attack, so I don't believe (there will be) acts of revenge,” he said.
Meanwhile the investigation into Friday's attack on Drottninggatan in central Stockholm continues. Police have interviewed more than 500 people over the attack. Last night a raid was carried out against a company in Sollentuna which is thought to be linked to the suspect, Expressen reports.
Police say their investigation has “strengthened” the suspicions against a 39-year-old Uzbek man in custody. Police official Jan Evensson told SVT on Sunday evening that he is “pretty convinced” that he is their man. “He will be locked up for the rest of his life,” Evensson predicted in the interview.
Another man was arrested on Sunday for alleged terror crimes, on a lower degree of suspicion.