In a dimly lit conference room on an upper floor of a Chicago mid-rise, an intricately detailed snapshot of American peril is being taken, minute by unsettling minute.

Reports from around the country — of gunshots, bomb threats, menacing antisemitic posts — flash across more than a dozen screens. A half-dozen analysts with backgrounds in the military or private intelligence are watching them, ready to alert any one of thousands of synagogues, community centers or day schools that appear to be at risk. Often, the analysts are the first to call.

This is the headquarters of the Secure Community Network, the closest thing to an official security agency for American Jewish institutions. There are other organizations that specialize in security for Jewish facilities, but none as broad as this group, which was created by the Jewish Federations of North America after 9/11. It has grown exponentially over the past five years, from a small office with a staff of five to a national organization with 75 employees stationed around the country.

What prompted its rapid expansion was the murder of 11 worshipers from three congregations by a hate-spouting gunman at the Tree of Life synagogue on Oct. 27, 2018, the deadliest antisemitic attack in American history.

The trial for the gunman, scheduled to begin on Tuesday at the federal courthouse in Pittsburgh, is taking place in a country that will be less shocked by any revelations than it might have been five years ago, given the prevalence now of mass shootings and incidents of antisemitism. The White House last week announced what it called the first-ever national strategy to counter antisemitism, involving multiple agencies and focusing on training and prevention.

But if Jews in America are less surprised by such incidents now, they have become, by grim necessity, far more vigilant.

The mass shooting in Pittsburgh was followed by arguably the most ambitious and comprehensive effort ever taken to protect Jewish life in the United States. In addition to bringing in more than $100 million dollars in federal grants to local Jewish organizations, the Jewish Federations of North America has raised $62 million with the ultimate goal of securing “every single Jewish community” on the continent.

There are now 93 Jewish Federations with full-time security directors, a more than fourfold increase over the past five years.

Local federations have long discussed security concerns with mayors and police chiefs, and some have paid for guards at schools and other places, said Eric Fingerhut, president of the J.F.N.A. But never, he said, has there been “this kind of comprehensive effort to say every institution in every Jewish community needs to be secured and connected to a best-practices operation.”

Overseeing much of this operation is the Secure Community Network. The group’s senior national security adviser, the man who designed much of the approach that it shares with local federations, is Bradley Orsini, a burly, gregarious former F.B.I. agent. In October 2018, he was the security director for the Jewish Federation of Greater Pittsburgh.

“The worst day of my professional career,” Mr. Orsini said in an interview at the group’s headquarters. He had been in charge of preparing the community for calamity, and it happened. But there was another way of looking at it, one that is the foundation of the work he does now: Had they not been taught the basic tactics of active-shooter response, the horror at Tree of Life would have been even worse.

“Bad things are going to happen,” Mr. Orsini said. “But we can give ourselves an edge.”

In a report released in March, the Anti-Defamation League counted 3,700 instances of antisemitic harassment, vandalism or assault around the country last year alone, the highest number in 43 years of keeping track. The F.B.I. has also found hate crimes on the rise; of religiously motivated hate crimes, nearly two-thirds were targeted at Jews.

The most terrifying of these have made national news, such as the hostage situation last year at a synagogue in Texas. In January 2022, a British citizen, apparently radicalized by Islamist extremists, took a rabbi and several others hostage. The hostages escaped unharmed — due in large part, the rabbi said afterward, to the training they had received from the Secure Community Network.

“It’s unfortunate that we’re growing, because the need is unfortunate,” Mr. Orsini said. “Everybody knows it’s not a matter of if. It’s a matter of when and where.”

When Mr. Orsini went to work at the Pittsburgh federation in 2017, Jewish people in the city and elsewhere were noting an ominous turn in the national rhetoric, in the undisguised hostility toward immigrants and dog-whistle warnings about “globalist elites.” But few saw imminent danger.

“When Brad started going out to our organizations, he said, ‘Do you get any threatening phone calls?’” said Jeff Finkelstein, the president of the Pittsburgh federation. “And they said, ‘Yes.’ ‘So what do you do?’ ‘We don’t do anything.’”

Mr. Orsini, who is not Jewish but was attuned to the menace of violent bigotry from his years on the civil rights squad in Pittsburgh’s F.B.I. office, devised a systematic approach to guarding Jewish institutions against attacks, which he called “the Pittsburgh model.”

He began by closely examining all of the Jewish facilities in the region and recommending security improvements, like planning escape routes or installing bullet-resistant glass. He set about strengthening ties with local law enforcement and encouraging people to report any sign of hate activity.

And he held more than 100 training sessions, including two at Tree of Life, where in 2017 a skeptical congregant named Steven Weiss learned the principles of “run, hide, fight.”

“We were just going through the motions,” Mr. Weiss, then a teacher, recalled. What was the point, he thought at the time. “Nothing is ever going to happen here.”

On a drizzly Saturday morning at the synagogue a year later, as he heard the gunfire in the hallway outside the chapel, Mr. Weiss scrambled to crouch behind a pew. Then he remembered Mr. Orsini’s words: “Don’t hide in plain sight. You’ve got to get out.” He saw another door and, with the gunshots growing closer, fled the room.

Active-shooter training is no guarantee against the kind of terror that unfolded on that day. But Mr. Weiss credits it with his survival.

The November after the attack, Lloyd Myers, a health care entrepreneur and philanthropist who worshiped for a time at Tree of Life, gathered a few dozen people for a brainstorming session.

“I started asking: ‘How could this happen?’” he said. “I’d ask my family, I’d I ask rabbis, I’d ask people with the Federation. And everybody said, ‘The reality is nobody’s watching our backs.’”

Mr. Myers’s health care technology business had specialized in gathering open-source data and scouring it for patterns or signs of trouble. He wondered if this expertise could be of use. Mr. Orsini told him about the Secure Community Network.

Mr. Myers’s epidemiological approach — of “looking at hate as a virus,” as he described it — has come to fruition in the conference room full of screens at the network’s headquarters.

Much of the analysts’ days are spent plumbing the sewers of the internet, sifting through posts doxxing prominent Jewish people or extolling violence, a noxious chore that one analyst referred to as “proactive threat-hunting.”

There are around 1,300 individuals in these channels whom the analysts watch particularly closely, sharing hundreds of disturbing finds with law enforcement which in some cases have led to arrests. But analysts said that antisemitic extremism is more decentralized than it was a few years ago, when the neo-Nazis who marched in Charlottesville in 2017 drew mainstream attention to more organized far-right groups.

White supremacy shows up now in racist fliers tossed into front yards, in small rallies that quickly form and dissipate and in torrents of vile chatter coursing through online forums. In some ways, one analyst said, it makes things even more dangerous, akin to the scattering of small, quasi-independent terror cells.

The network is planning to operate a temporary outpost in Pittsburgh during the shooter’s trial, which will largely revolve around the question of whether he should be put to death.

The network’s director, Michael Masters, a Harvard Law grad who served in the Marines, said that many Jewish communities he spoke with saw the attack in Pittsburgh at first as a tragic anomaly, rather than a sign of a new normal. But the shooting exactly six months later at a synagogue in Poway, Calif., in which the assailant named the Pittsburgh attacker as an inspiration, unraveled that notion.

“That was the moment where Brad and I saw a shift,” Mr. Masters said. “Even if you got that question still — ‘Well, I don’t know that it’s going to happen here’ — you could say, ‘Pittsburgh, Poway. We’re not going to choose the time and place.’”

The need for a newfound vigilance has largely been acknowledged, but there are still those who seem resistant. Mr. Weiss learned this when he left Pittsburgh and joined a new congregation in Lebanon, Pa., where he immediately pointed out shortcomings in the synagogue’s security.

The rabbi there, Sam Yolen, said many members readily understood Mr. Weiss’s warnings — particularly the young, who had seen the hate metastasizing online, and the very old, who had lived at a time when antisemitism was a fact of everyday life.

But some, he said — those who had come of age believing that they could live as Jews in America largely unexposed to threats or danger related to their identity — had required more convincing. “People who might have grown up with America’s promise of a white picket fence,” Rabbi Yolen said, are having to learn that “that was the exception. Not the hate that we are experiencing now.”

The hostage situation in Texas last year was one of the more recent reminders of this new normal. After an 11-hour standoff at the synagogue, the rabbi, who had recently undergone training with the Secure Community Network, threw a chair at the attacker, giving the hostages a chance to escape. That chair now sits on a low platform in the Chicago headquarters.

Beside it is a smaller chair, the vinyl faded and pockmarked with holes. It is from Tree of Life.