The National Bureau of Investigation says passwords posted online late Saturday night appeared to be connected to a previous incident, in which the personal data of nearly 16,000 Finns were hacked into and made public.
They have already interviewed suspects in connection with the latest case. Police are trying to discover if the data were acquired by illegal means.
Data protection experts and police recommend changing passwords regularly, regardless of whether or not personal details have been leaked recently.
Erkki Mustonen of the computer security firm F-Secure says that a good password should be difficult to guess, and that swapping letters for numbers that look similar helps strengthen it.
He also recommends keeping the same password for a maximum of six months. Ideally passwords should be changed every three months.