These people may have established links with organized crime while in prison, YLE outlet reported, citing Supo Director Antti Pelttari.
"While in prison, these people may not necessarily have changed the way they think, unfortunately," Pelttari said at a security seminar at the Parliament on Wednesday.
The Supo director added that no factors would allow to lower the threat of terrorism at the moment. The intelligence agency has kept its assessment of threat level as "elevated" since 2017.
In the last several years, a number of terrorist attacks have been carried out in Europe. In Finland, 10 people were stabbed in the city of Turku, two of them died, in August 2017.