The shooter was identified as Frazier Glenn Miller, a 73-year-old white supremacist. The attack illustrates the dilemma of how best to protect Jewish institutions from the threat of deadly violence by extremists acting alone.

“Lone wolves are really by far the most dangerous phenomenon. They are vastly more difficult to stop in advance of their actions,” said Mark Potok, the publications director for the Southern Poverty Law Center. “You can’t simply follow around all the people in the United States who have noxious views.”

Vigilance on the part of communal institutions is key, said Paul Goldenberg, who directs the Secure Community Network, the security arm of national Jewish groups.

“The only way is to stop the lone wolf is prevention and hardening a soft target,” Goldenberg said.

Miller, who is scheduled to appear in court on Monday, is suspected of killing a man and his grandson on Sunday in the parking lot of the Jewish Community Center of Greater Kansas City in Overland Park, Kan., and then shooting to death a woman at Village Shalom, a Jewish assisted-living facility a few blocks away, where she was visiting her mother.

After Miller was placed in a police vehicle, he was heard to yell “heil Hitler.”

The family of the victims killed at the JCC were William Lewis Corporon, a retired physician, and his 14-year-old grandson, Reat Griffin Underwood. Corporon and Underwood, members of an area Methodist church, were hit by bullets as they were in a car. Underwood, an aspiring singer, was at the JCC for a talent show, family told local media.

The third victim, Terri LaManno, a Catholic mother of two, was killed in the Village Shalom parking lot.

This was not the first time a JCC has been targeted by a lone gunman.

In 2006, Naveed Afzal Haq, motivated by anti-Israel views, killed one woman and wounded five others when he attacked the Seattle Jewish Federation building.

In 1999, white supremacist Buford Furrow wounded five people, including three children, when he opened fire on the North Valley Jewish Community Center in suburban Los Angeles and shortly after killed a mail carrier.

The Southern Poverty Law Center was the first to identify the gunman as Miller, of Aurora, Mo. It said he was the grand dragon of the Carolina Knights of the Ku Klux Klan in the 1980s and subsequently a founder of the White Patriot Party. The center said he served three years in prison on weapons charges and for plotting the assassination of its founder, Morris Dees.

Miller had not been involved in criminal activity for decades, but he kept his views known and publicized them as avidly as he could. He maintained a website, www.whty.org (for “whitey’) and posted links to his media appearances, including one on a black radio show.

In 2012, he appeared on a panel of extremists organized by a professor at Missouri State University and reveled in the encounter. In a post on the Vanguard News Network, a white supremacist site, he describes sparring with Jewish students from the audience, whom he described as “two kikes.”

Mark Pitcavage, the director of investigative research for the Anti-Defamation League, said that lone wolves tend to operate on the margins of extremist communities, which makes it harder to detect when they may be plotting actions. This was true of Miller, who had alienated much of the movement in the 1980s when he got a reduced sentence in exchange for testifying against co-conspirators.

There are times, Pitcavage said, when monitors are able to detect planning for violence. Often extremists will report in online forums private exchanges with individuals seeking co-conspirators for a violent act.

“When we see extremists start warning other extremists about someone, we pay attention,” Pitcavage said. “The way the vine works, they think. ‘He’s a government plant who’s trying to get me in trouble.’ They have a skewed reaction to it, but nevertheless they have a reaction. We have learned when we see those sorts of things to take them seriously.”

The ADL passes on such information to law enforcement.

In other instances, there are signs that a poster is a “powder keg,” Pitcavage said.

Monitors, he said, look for “a long series of posts expressing aggravation, ‘Something has to be done, it’s time to do something,’ they may say things to other people trying to get people involved.”

Potok of the SPLC said that extensive posting on such sites is not in of itself necessarily an indicator of such violence. Wade Michael Page, a white supremacist who in 2012 killed six people at a Sikh temple in Oak Creek, Wis., had been prominent among online extremists but had not exhibited typical signs of an imminent attack.

“We had been following Page for 12 years,” Potok said. “There was no indication that he had finally decided to start shooting.”

Other times, Pitcavage said, lone wolves operate completely under the radar, with no communications whatsoever preceding an attack. White supremacist Keith Luke killed two West Africans in the Boston area in 2009 and was on his way to attack a synagogue when police stopped him.

“No one had ever heard of Keith Luke before,” he said. “After his arrest we discovered he had spent countless hours watching white supremacist videos on YouTube.”

Other lone wolves embrace the status because of its utility, Pitcavage said, noting that Anders Behring Breivik, who killed 77 people in bombings and shooting in and near Oslo on July 22 2011, had assembled data showing that lone attacks were more successful.

“The more steps there are, the more people there are,” the likelier it is that the plot will be leaked, Pitcavage said.

Goldenberg, the Jewish security official, said that it has become easier for potential assailants to surveil Jewish targets because of information that’s easily accessed on the Internet. It’s not clear what drew the assailant to the Kansas City JCC, Goldenberg said, but it was notable that there were at least two events that had been publicized and were likely to draw crowds: a play and the singing audition.

It was a tough balance, said Karen Aroesty, the ADL director in St. Louis, Mo., who was in touch with the Kansas City community and law enforcement in the wake of the attack.

“You want communities to spread the word about the activities they are doing, balancing that with the kind of security that protect against” potential assailants, she said. “How do they pitch security really strongly while being warm and welcoming? That’s a tough balance.”

Goldenberg noted the importance of training members of the Jewish community.

“It is empowering members of the Jewish community through education and knowledge, how to identify a suspicious person, how to face an active shooter,” he said.

With the caveat that he was not yet fully apprised of how the JCC shooting went down, Goldenberg noted that the building immediately went into lockdown, that the assailant struck from the rear — away from security officers who would have been manning the front — and that he did not breach the back door.

“Sometimes the best defense is a locked door,” he said. “The best defense is having a plan for a lockdown and keeping the individual outside. The individual did not gain entry to the building and that undoubtedly saved many lives.”

The Jewish Federations of North America, a leader in obtaining Department of Homeland Security funding for security measures for Jewish buildings, said the Overland Park shootings underscored the need for the program.

“The horrific shootings in Kansas City emphasize the fact that Jewish communal institutions have been the victim of an alarming number of threats and attacks,” William Daroff, JFNA’s Washington director, wrote in an email. “Due to those threats, the federal Nonprofit Security Grant Program has provided millions of dollars to assist nonprofits in upgrading their security capacity.”

President Obama, who in a statement said the attack was “heartbreaking,” pledged federal resources to the investigation.

“I have asked my team to stay in close touch with our federal, state and local partners and provide the necessary resources to support the ongoing investigation,” he said.