The Helsinki Police Department's youth crime division suspects a group of underage youths of aggravated robbery.
Authorities suspect the youngsters of using a website to lure a middle-aged man for a sex-related meeting and then rob him.
Police said that when the man arrived at the agreed meeting place, the youths threatened him with a machete knife and then robbed him. They are suspected of taking the man's money.
One underaged boy was detained on suspicion of aggravated robbery, while three other boys and one girl are suspected of participating in carrying out the crime.
Police said they suspect the girl and one of the boys made arrangements with their middle-aged target on a website called seksitreffit.fi (roughly translated to "sex dates"). Police also suspect three other boys of taking part in the robbery itself.
Three of the suspects were known to police from a similar incident in the southern municipality of Järvenpää. In that incident, the suspects created a fake online profile and email address on the site as a way to reach out to potential targets.
Police said interrogations have revealed that several similar robberies may have been carried out in the capital area and eastern Uusimaa. So far, however, police said they have not yet received reports about such crimes.
Police said there should be a low threshold for potential victims to report such crimes.