Members of the Toronto synagogue Adath Israel Congregation opened a plain-looking envelope to find a swastika and the words "Jewry must perish" on Monday, an occurrence that human rights organization B'nai Brith Canada says happens all too frequently.

The letter, featuring a full-page drawing of a swastika inside what appears to be a bleeding Star of David, left members of the congregation afraid for their safety, B'nai Brith Canada representative Marty York said.

York said the woman who handled the letter was "terrified" it could contain something poisonous. 

B'nai Brith Canada reported that the same letter had been delivered to synagogues in Montreal, Hamilton and Edmonton. They're aware of five letters in total, but York suspects there may be more.

"It was an eight on the frighten scale," York told CBC Toronto. "But I've seen nines and 10s, all the way down the ladder, almost every day." 

The organization reported last spring they'd heard about more than 1,700 anti-Semitic incidents across Canada last year, the highest number yet.

The group has been tracking anti-Semitic incidents since 1982, which is based on police data and calls to the group's anti-hate hotline. Of the 1,728 incidents, 490 occurred in Ontario.

B'nai Brith Canada CEO Michael Mostyn encouraged vigilance against Nazi symbolism in a press release.

"Sadly, we've seen the swastika make something of a comeback this year, defacing the walls of high schools, university campuses, and public property," Mostyn said.

Prime Minister Justin Trudeau said in a tweet: "These recent acts of hatred and anti-Semitism have no place in our country and we will not tolerate it."

A spokesperson for Toronto Police Services said officers investigated the letter on Monday and will likely proceed by assigning a detective to the case.

With files from The Canadian Press