Studies find that many of them connect with like-minded believers online and broadcast their ideology and intentions before they act.

Police tape is seen outside Pittsburgh's Tree of Life Synagogue on October 28th, 2018, after a shooting there left 11 people dead. (Photo: Brendan Smialowski/AFP/Getty Images)

America is reeling as the country grapples with the homegrown terrorist acts and hate crimes of the last few weeks. First, Democratic politicians and donors and the network CNN received homemade pipe bombs in the mail. Then, last weekend, a gunman entered a synagogue in Pittsburgh, while people were gathered for a baby-naming ceremony. He killed 11 people and injured an additional six. The federal criminal complaint against him says he afterward told a SWAT team member he wanted "all Jews to die."

Investigators are still collecting evidence in these cases, and it's possible that details will emerge that place the suspects within larger terror or hate networks. However, so far, what the public knows is that only one person has been charged with each crime. If the suspects turn out to be so-called "lone wolf" terrorists, they'll join many others. As David M. Perry argued in Pacific Standard earlier this year, a number of recent white supremacist lone-wolf killers have subscribed to related, hateful ideology. "They're reading the same websites, talking to each other, and killing the same targets," as Perry put it. "The lone wolves are actually a pack."

The research bears him out. Although, by definition, lone-wolf terrorists don't belong to formal organizations, a few large recent studies have sought to find trends and commonalities between them. Here are some highlights on what we know about lone perpetrators of terror.

Lone Wolves Are Motivated by a Variety of Causes

These include white supremacy—and radical Islam. Although al-Qaeda is best known for more organized attacks, including the September 11th attacks on the United States, many lone wolves are inspired by al-Qaeda without formally joining the organization. In a 2014 study called "Bombing Alone," which defines lone wolves to include certain isolated pairs of terrorists, 43 percent of the terrorists analyzed were inspired by al-Qaeda, while 34 percent were right-wing extremists. In fact, both adherents to al-Qaeda and white supremacy have advocated for acting independently, to make it harder for law enforcement to detect terrorism plans.

"Bombing Alone" found very few far-left lone wolves, although it did include a category for single-issue terrorists, who might believe in far-left or far-right causes, including ecoterrorists and anti-abortion extremists. Fewer than one in five lone-wolf terrorists in that data set were motivated by single issues.

They're Frequently White, Unemployed, Single Men With Previous Criminal Convictions

That's how The Age of Lone Terrorism, a book based on Federal Bureau of Investigation data spanning from 1940 to 2016, puts it. (Note The Age of Lone Terrorism defines "lone wolf" slightly differently than "Bombing Alone," so their findings are not exactly comparable.)

Among the "Bombing Alone" data set, which was based on media reports dating after 1990, 97 percent of attackers were male. About 40 percent were unemployed. Forty-one percent had been convicted of a crime before, a greater rate than American adults in general, about one-third of whom have either been convicted or arrested (but not necessarily convicted) on a felony charge. Twenty-six percent of lone terrorists had served time in jail, and half of them were radicalized there.

But It's Otherwise Hard to Build a Stereotypical Profile of Them

Lone wolves vary enough that you can't profile them accurately, write the authors of "Bombing Alone," a team sponsored by the U.S. Department of Homeland Security and coordinated by the United Kingdom's immigration and anti-terrorism agency, the Home Office. In any case, you wouldn't want to: The vast majority of folks who fit in the above demographic profile are not terrorists, and many terrorists would escape notice if law enforcement leaned too much on stereotypes.

They May Be 'Lone Wolves,' but They're Not Isolated

Despite not formally belonging to terror organizations, 35 percent of the terrorists in the "Bombing Alone" study had talked with other believers online, and 48 percent had met others subscribing to their ideology in person.

They Tell Other People About Their Beliefs and Intentions

In nearly 60 percent of the cases in "Bombing Alone," the terrorists had made public statements about their beliefs. In more than 80 percent of cases, friends or family knew about their grievances with society. The Age of Lone Terrorism finds that nearly all lone-wolf terrorists broadcast their intentions somehow.

These last characteristics have certainly been true of the men charged in the pipe-bomb mailings and the Pittsburgh shooting, both of whom are reported to have posted online about far-right conspiracy theories. The alleged Pittsburgh shooter had also posted anti-Semitic threats.