Eija Palosaari, a leading Finnish crisis psychologist told Yle's TV 1 morning show on Monday that processing traumatic events is different for everyone.
"In many cases," Palosaari said, "fear can sneak up on a person."
"[Fear] is a natural emotion and it's not a good idea to try to push feelings aside by telling yourself 'don't be afraid,'" she said.
How people are affected by traumatic events varies from person to person, Palosaari said, and really depends on whether they have been able to fully process the trauma they've experienced.
Palosaari said violent events can spark recurrent and frightening dreams or thoughts, and her advice to people who experience such effects is to confront them.
"The rule is that you should look at pictures [or reminder of the traumatic event] in order to be able to process the emotions," she said.
But if viewing the images becomes too overwhelming, she said it may a good idea to seek help from a trauma counsellor.
How long can these feelings last?
Palosaari says that the amount a time it takes a person to get over a traumatic event varies greatly. That emotional process, she says, tends to be gradual.
There were several hopeful stories amidst all of the violence and sadness on Friday - about heroes who stood up to the attacker and helped the wounded.
Are those people different from others? Don't heroes get scared?
"People who act heroically and selflessly are helped in a way because they're in a state of shock themselves where they lose contact with their own feelings of fear and timidity," Palosaari said. "Afterwards they often are themselves amazed by how they acted."
Palosaari was recipient of the Finnish Psychological Association's psychologist of the year award in 2008.