(CNN) In recent years, mass shooters have increasingly targeted a range of venues -- churches, synagogues, grocery stores, movie theaters -- that have minimal security.
That's no surprise, experts who study violence and mass shootings say. These attackers, including the one last weekend in Buffalo, choose so-called "soft targets" where people are largely unprotected and where their bullets will have maximum impact.
Before 2017, there was only one recorded mass shooting at a supermarket in the US, according to Jillian Peterson, a criminologist at Hamline University in Minnesota and founder of The Violence Project, a nonpartisan research center. That was in 1999, when a gunman dressed in camouflage killed four people at a Las Vegas grocery store.
But in the last three years gunmen have targeted a kosher market in Jersey City, New Jersey, a Walmart in El Paso, a supermarket in Boulder, Colorado, and now a supermarket Saturday in Buffalo, where an 18-year-old White gunman is suspected of gunning down 10 shoppers, most of them Black.
These attacks follow other recent mass shootings at places of worship, including a Black church in Charleston, South Carolina, a synagogue in Pittsburgh and a Baptist church in Sutherland Springs, Texas.
As law enforcement grapples with this violent extremism, experts say trying to secure thousands of such soft targets nationwide would be almost impossible. Javed Ali, a former top official at the FBI and Department of Homeland Security, describes it as "exponentially more cumbersome" than the efforts that went into protecting critical infrastructure after the September 11, 2001, attacks.
"From what we know about mass shooters, they tend to pick targets that allow them the best chance of success. The combination of target vulnerability plus attack capability plus perceived impact usually drives how these events unfold, although there is no clean scientific or mathematical algorithm that can precisely determine when and how attacks happen," says Ali, an associate professor at the University of Michigan.
"That's why trying to 'harden' numerous soft targets across the country would be a difficult task."
Tung Yin, an expert on national security and terrorism, uses a balloon analogy.
"Trying to stop such attacks by hardening one set of targets is like squeezing one of those balloon animals: the air just goes to a different part of the balloon," says Yin, a professor at Lewis & Clark Law School in Portland, Oregon. Would-be terrorists will just go looking for a softer target, he says.
There are no easy solutions. But experts say a better approach would be trying to identify potential mass shooters online before it's too late.
Mass shootings tend to breed copycats, fueled by media coverage and the notoriety such attackers get, Peterson says. (CNN and the Gun Violence Archive define a mass shooting as one that injures or kills four or more people.)
She describes it as a social contagion.
"Once one shooting gets a lot of media attention, it becomes a blueprint that future shooters follow," Peterson says. "Individuals who are on the edge and looking for answers see themselves in other perpetrators, and they are looking for the same fame and attention."
After the Buffalo massacre, authorities found evidence indicating the alleged shooter had studied previous hate attacks, said an official familiar with the investigation. In a hate-filled document, the Buffalo suspect mentioned a mass shooter who killed dozens of people in 2019 at a mosque in Christchurch, New Zealand.
The fact that such attackers operate alone also makes it harder to preemptively stop them, experts say.
"We are not dealing with organized threats. Lone wolf attackers can mobilize undetected and without breaking laws up until the day of," Ali says. "How broadly would you want to impose security measures? They would have to be so vast and pervasive."
Many recent shooters were apparently motivated by hate towards Black people, Hispanics or Jews.
Grocery stores in certain neighborhoods are frequented by a specific demographic, which can make them a target for hate-fueled violence, says Peterson, who's written a book titled "The Violence Project: How to Stop a Mass Shooting Epidemic." Examples are Asian stores and kosher supermarkets, she says.
The alleged gunman in Buffalo told authorities he was targeting Black people, according to an official familiar with the investigation.
Investigators believe the gunman was in Buffalo a day before the shooting to do reconnaissance at the market, Buffalo Police Commissioner Joseph Gramaglia told CNN. During his visit, he noted how many Black and White people were in the store.
Other recent shootings appear to have been inspired by similar hate.
Investigators say a deadly 2019 attack on a kosher market in Jersey City, New Jersey, was fueled by anti-Semitism.
"We believe the suspects held views that reflected hatred of the Jewish people, as well as a hatred of law enforcement," New Jersey Attorney General Gurbir S. Grewal said at the time.
In 2018, a white supremacist gunned down 11 Jews at a synagogue in Pittsburgh. While receiving medical care after the shooting, he told a SWAT officer that he wanted all Jews to die, according to a criminal complaint.
The man charged with killed 23 people at a Walmart in El Paso in 2019 told police he was targeting Mexicans, according to an arrest affidavit. Investigators allege he wrote a document describing his hatred for immigrants and Latinos, and said he wanted to stop a "Hispanic invasion" of Texas.
The Center for Strategic and International Studies said white supremacists and other far-right extremists were to blame for 66% of domestic terrorist attacks and plots in 2020.
And in a 2020 document, Homeland Security officials said "racially and ethnically motivated violent extremists — specifically white supremacist extremists — will remain the most persistent and lethal threat" in the country.
It's normal to see extra security measures and calls for tightened security after shootings, Yin says.
But he says it's important to evaluate such decisions from an economist's perspective.
"For every potential security measure, what's the marginal gain and what's the marginal cost?" he asks.
Adding checkpoints and metal detectors to enforce a gun-free zone could lead to long security lines, which would create a new vulnerability point for attacks, he says. Such measures could also turn a routine trip to the store into a hassle.
"It would add so much time, at some point it will strangle the livelihood of society," he says.
Adding armed security guards also isn't necessarily effective, experts say.
In Buffalo, security guard Aaron Salter fired his gun at the shooter, who was wearing body armor. The gunman was unharmed and returned fire, killing Salter.
"Often our reaction after these awful tragedies is to add more armed security, which does influence our overall fear, anxiety, and sense of safety in the world," Peterson says. "We have done prior research showing that armed officers often don't minimize the number of casualties ... because the shooters are usually suicidal and plan to die in the act. That makes these tragedies harder to prevent."
Pete Eliadis, a former law enforcement official and founder of a security consulting firm, says most people in the US won't tolerate strict security at places like stores. However, he believes companies can help reduce risks by making security training a higher priority.
"Active threats are no longer a rarity but something that organizations should train their employees (for)," he says. "Organizations train their employees for a variety of things -- sexual harassment, cyber security ... yet the thing that they can actually die from, they don't prioritize."
Peterson emphasizes the importance of keeping firearms out of the hands of dangerous people through "red-flag" laws and universal background checks.
Ali says another approach would be to identify extremist threats online before they lead to violence. Constitutional protections make it difficult for law enforcement to act on hate speech -- but private companies can, he says.
"Monitoring hate speech online by the government is a slippery slope, given First Amendment protections on free speech," he says, "which is why the technology companies who own and operate the platforms on which this content resides are in a better position to detect, flag, and remove it than the government."
"We need solutions that would be more efficient than trying to over-secure vast places," Ali says. "I'd like to see us get more upstream, identify who these people are and ... do a better job of weeding these people out. If we're waiting for the last minute, there's only so such we can do."