Like other Jewish populations around the world, Chicago's community is no stranger to threats of violence.

So when the Department of Homeland Security contacted the city's Jewish institutions last week to say that two bombs had been found in printer cartridges bound for local synagogues, there was little surprise.

"When we got the call we knew what to do," said Michael Kotzin, executive vice president of the Jewish Federation of Metropolitan Chicago. "We weren't surprised. But there was a lot of curiosity about the details."

Initially, federal investigators suggested the bombs were an al-Qaeda strike directed at Chicago.

But this week officials said the packages were probably primed to blow up in mid air, en route from Yemen.

Mr Kotzin said it was reassuring to know the bombs were not supposed to hit Jewish targets.

"But it doesn't change the fact that specific packages were addressed to Jewish institutions and it doesn't change the reality of the world we live in," Mr Kotzin said.

The seriousness with which the threat is being taken was underlined on Tuesday when two hundred Jewish leaders were given a security briefing in a conference call from the FBI.

The last serious threat to Chicago's Jewish population, which numbers about 270,000, was in 1999.

In July of that year, Benjamin Nathaniel Smith, a white supremacist, shot and wounded six Orthodox Jews before killing an African-American basketball coach and a Korean student.

Rabbi Michael Balinsky, executive vice president of the Chicago Board of Rabbis, said: "We are already security conscious. This has raised another issue to think about."

Meanwhile, the Jewish Federation of Metropolitan Chicago advised synagogues to intensify screening procedures and to take added precautions when monitoring buildings.

Though investigators did not reveal the names of the synagogues involved in the bomb plot, Rabbi Larry Edwards of Or Chadash said he believed his congregation was one of the targets.

Or Chadash, an LGBT congregation, rents space in the Emanuel Congregation synagogue on Chicago's lakeside.

Rabbi Edwards said Congregation Emanuel's rabbi, Michael Zedek, had been informed by federal officials that the threat to his shul was credible.