Strict secrecy surrounded the fortress for a very long time, and there were several attempts made by foreign powers to gain knowledge of various kinds of information. One early possible attempt at espionage were the Russian sawfilers who travelled through Sweden, mostly in Norrland during the last years of the 19th century and the years leading up to World War I. A large part of the Swedish population believed that the sawfilers, no more than 300 in total in Sweden, were spies hired by the Okhrana, the Russian secret police. Some sources are of the belief that the sawfilers were never sent out on specific missions but instead acted as unknowing spies, only having contact with the Okhrana who interrogated them regarding their experiences when they returned to Russia. Other sources claim that at least some of the sawfilers that the Okhrana found most useful were educated in the art of espionage, and were in fact given specific missions, as well as being paid 300 rouble per season for their work. Another form of this kind of legal espionage is believed to have been conducted by the German Wandervogel movement during the late interwar period.