The Lake Bodom murders is one of the most infamous unsolved homicide cases in Finnish criminal history. Sometime between 4:00 a.m. and 6:00 a.m. (EET) on 5 June 1960, at Lake Bodom in Espoo, Uusimaa, Maila Irmeli Björklund and Anja Tuulikki Mäki (both 15) and Seppo Boisman (18) were killed by stabbing and blunt-force trauma to their heads while sleeping inside a tent. The fourth youth, Nils Gustafsson, then aged 18, was found outside the tent with broken facial bones and stab wounds.
https://en.wikipedia.org/wiki/Lake_Bodom_murders