The prosecutors' original indictment accused the man, whose identity remained under gag order until recently, of placing some 2,000 threatening calls to mainly Jewish institutions in the US and internationally, Israel's Justice Ministry statement said.
"In a short time, a large number of Jews are going to be slaughtered. Their heads are going to be blown off," one of the threats read, according to a recording obtained by the Jewish Telegraphic Agency.
Israel's Haaretz newspaper said Friday the net started to tighten after a threat in New Zealand in 2016 when police found the threat came from an Israeli IP address.
Using an antenna, the suspect allegedly accessed other people's computers to commit the crimes, the newspaper said, leading police to question a number of innocent suspects before eventually netting him.
Kadar's conviction might assuage fears of rising anti-Semitism in the United States and around the world, knowing that the bomb threats were a hoax and people were not in immediate danger as they originally feared.
However, US Jewish organizations tried to downplay the political fallout, saying the arrest did not end legitimate fears.
"No arrests have been made in three cemetery desecrations or a series of other anti-Semitic incidents involving swastika graffiti and hate fliers," the Anti-Defamation League's CEO Jonathan Greenblatt said in a statement.
The fact that a young Jewish man incited widespread fear among Jews appeared to vindicate white nationalists and far-right conspiracy theorists, who have long claimed that such threats are part of a Jewish plot.
"The outcome of this young man’s actions is that the classic anti-Semitic conspiracy theory will be given a tailwind -- the Jews portray themselves as victims but are orchestrating the supposed attacks," an article in Israel's Yediot Aharonot newspaper said.