The horrific killing of 11 worshipers at a Pittsburgh synagogue has taken a festering problem out of the shadows and put it in the spotlight: anti-Semitism is on the rise in America.

Bomb threats, menacing messages on social media and assaults on Jewish Americans have become increasingly common in recent years, according to researchers and federal data.

Even before the attack – believed to be the deadliest in U.S. history targeting American Jews – violence and harassment of Jewish people and institutions was rising sharply, coinciding with a moment when American politics has become sharply divided.

There were at least 1,986 such incidents motivated by anti-Jewish bias – including physical assaults, vandalism and attacks on Jewish institutions – in 2017, a 57 percent spike in incidents over the year before, according to the Anti-Defamation League, a group that tracks and fights anti-Semitism.

The ADL also published a report this month that found an alarming increase in the amount of anti-Semitic content on social media in the lead-up to next month’s midterm elections.

“Prior to the election of President Donald Trump, anti-Semitic harassment and attacks were rare and unexpected, even for Jewish Americans who were prominently situated in the public eye,” authors Samuel Woolley and Katie Joseff wrote.

“Following his election, anti-Semitism has become normalized and harassment is a daily occurrence. The harassment, deeply rooted in age-old conspiracies such as the New World Order, which alleges that an evil cabal of Jewish people have taken autocratic control of the globe, and Holocaust imagery – faces placed inside Nazi concentration camp ovens or stretched on lampshades – shows no signs of abating.”

Authorities on Saturday charged Robert Bowers, 46, with murder, hate crime and firearms offenses for the attack on the Tree of Life Synagogue. Before surrendering to police, the suspect ranted that Jews were “committing genocide to my people” and “I just want to kill Jews,” according to a criminal complaint.

The suspect also appeared to have posted virulently anti-Semitic messages on a social media platform popular with far-right extremists, including an angry diatribe apparently written shortly before the attack.

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Vice President Mike Pence told NBC News that "we need to be very careful" before connecting political debate and "the kind of violent behavior we witnessed in Pittsburgh," the "threats of violence against prominent Americans that we witnessed in the pipe bombs" mailed to prominent Trump critics last week, or other recent mass shootings.

A 97-year-old, an elderly wife and husband: The 11 Pittsburgh victims

Woolley and Joseff’s analysis of 7.5 million postings on Twitter between Aug. 31 to Sept. 17 found a significant share of anti-Semitic messaging on the social media site they reviewed leading up to the midterms – as many as 30 percent –appeared to come from automated accounts, or bots.

The ADL researchers, as well as Jonathan Albright, director of the digital forensics initiative at the Tow Center for Digital Journalism at Columbia University, also have found that George Soros, the billionaire Hungarian-American and liberal philanthropist, has increasingly become the target of some of the worst anti-Semitic vitriol on social media platforms.

Soros, who is Jewish and a backer of Democratic Party candidates, is frequently framed by right-wingers as being responsible for unfounded conspiracies. Trump has accused Soros of paying demonstrators who protested in Washington against the confirmation of Justice Brett Kavanaugh.

Soros was one of a dozen high-profile Trump critics targeted last week in a series of attempted bombings authorities said was carried out by a Florida man who was arrested Friday.

Alexander Soros, the son of George Soros, wrote in a New York Times op-ed last week that his father had long been targeted by anti-Semitic attacks from white supremacists.

“But with Donald Trump’s presidential campaign, things got worse,” wrote the younger Soros, who also noted that Trump’s final TV ad of the 2016 campaign disparagingly featured his father as well as prominent Jews Janet Yellen, chairwoman of the Federal Reserve, and Lloyd Blankfein, chairman of Goldman Sachs. “A genie was let out of the bottle, which may take generations to put back in, and it wasn’t confined to the United States."

After Saturday’s mass shooting, Trump condemned the attack as an "anti-Semitic act" and “pure evil.”

"There must be no tolerance for anti-Semitism in America or for any form of religious or racial hatred or prejudice," Trump said.

Still, some lawmakers questioned whether Trump’s rhetoric was stoking the vitriol.

"This president's modus operandi Is to divide us,” Rep. Adam Schiff told CNN’s State of the Union on Sunday. “It's not enough that a day on a tragedy he says the right words if every other day of the year he's saying things to bring us into conflict with one another.”

Sen. James Lankford, R-Oklahoma, defended the president and noted that the suspected gunman had criticized Trump as a “globalist.”

"Now President Trump and his rhetoric is very direct, but I don't see how you connect President Trump to a person who's deranged going into a synagogue,” Lankford said on CBS' Face the Nation. “He's been very clear about anti-Semitism, as well as all of us have been. That is a sick, vile thing."

Federal data show that 54.4 percent of hate crimes committed in the U.S. in 2016, the last year for which FBI data are available, was motivated by the perpetrators with anti-Jewish bias.

In August 2017, dozens of white supremacists took part in a Unite the Right rally in Charlottesville, Virginia, where marchers shouted the Nazi-era salutation "Sieg Heil" and chanted “Jews will not replace us.” An Ohio man was charged with hate crimes after driving his vehicle into a group of counterprotesters at the Charlottesville rally, killing 32-year-old Heather Heyer.

Last year, Evansville, Indiana’s Temple Adath B'Nai officials reported that someone had fired a bullet through a window of their house of worship. No one was hurt in the incident, which came a day after legislation died in the Indiana Legislature that would have allowed judges to imposes stiffer sentences for crimes motivated by race, religion, sex, disability, gender identity or sexual orientation.

Separately, dozens of tombstones were overturned in Jewish cemeteries in Missouri and Pennsylvania last year.

In Pittsburgh's Squirrel Hill neighborhood, the center of Jewish life in the Rust Belt city, the community was cognizant that anti-Jewish fervor was something that needed to be taken seriously.

The Jewish Community Center and a nearby school recently had active-shooter drills, said Rabbi Amy Bardack, who was attending services at nearby Beth Shalom synagogue when she heard about the shooting. Anti-Semitic acts had taken place in community before, but not anything that caused physical harm to people, she added.

"October 27th will forever be a line in the sand of before and after," Bardack said.

The suspected Pittsburgh gunman had a long and troubling online history and at one point turned to biblical passages to try to buttress his anti-Semitic beliefs.

On his web page, Bowers featured a paraphrase of a verse from the Bible as “jews are the children of satan. (john 8:44) --- the lord jesus Christ is come in the flesh.”

While this portion of the Book of John has been used over the centuries as an excuse for attacking Jews, that is a flawed reading of it, said Sandra Schneiders, an emeritus professor of the New Testament at the Jesuit School of Theology in Berkeley, Calif.

Jesus is not talking about all people who are Jewish, Schneiders said.

“He is speaking to a particular group of Jews in Jerusalem. He’s clearly arguing with the authorities,” Schneiders, author of “Written that You Might Believe: Encountering Jesus in the Fourth Gospel.”

“He says that they say they are the descendants of Abraham, but they are trying to kill him, so they are showing their true parentage which is determined by what you do.” And that true parentage is Satan, Schneiders said.

“This man who shot all of these people, someone needs to say to him: ‘You claim to be a follower of Jesus, but this is not what Jesus said. You’re revealing your true parentage by your actions.”

Contributing: William Cummings