Finnish police have informed several members of the country’s Jewish community that a neo-Nazi activist has been gathering information about them.

Police sent a letter to an undisclosed number of individuals after finding details about Finnish Jews on a USB stick in the possession of a man arrested two months ago, according a report Friday in the Israeli daily Ma’ariv.

The man was taken into custody in connection with a stabbing that took place at the library of Jyvaskyla, a city 170 miles north of Helsinki, the Finnish capital. When police searched his apartment they found the USB stick containing a folder named “Jews.” It contained 300 photos and information, mostly about teenagers.

The name of the suspect was not disclosed as it is subject to a gag order. Finnish Jews have been instructed to exercise caution in revealing information about the events on Facebook and other social networks.