Dylann Roof. Adam Lanza. James Holmes. Jared Loughner. Eric Harris and Dylan Klebold.
Their names stir painful memories and conjure images of hate and violence. The killers have other characteristics in common too: They either were, or are, young, white and male.
Well, it's complicated. Analysts say a variety of factors could be at play.
"What made Dylann Roof do what he did? You know, I mean, that's the question, isn't it? I don't know," Jeffrey Swanson, professor in psychiatry and behavioral sciences at Duke University, told CNN's Don Lemon.
"There are pieces of the puzzle, and you put them all on the jigsaw board, and you're still going to have a big hole there. What I do know is that violent behavior -- whether it's serious violence or minor violence in populations -- is never just one thing. It's not a one thing problem. It's going to be an accumulation of things, kind of a whole cocktail of factors," he said.
Here's a look at some of those characteristics:
According to data compiled by Mother Jones magazine
, which looked at mass shootings in the United States since 1982, white people -- almost exclusively white men -- committed some 64% of the shootings.
The examples are infamous. Roof and the Charleston church massacre
. Lanza and the shooting at Sandy Hook
Elementary School in Newtown, Connecticut. Holmes
and the slaughter inside an Aurora, Colorado, movie theater.
Black people committed close to 16% of the mass shooting Mother Jones looked at, while Asians were responsible for around 9%. People identified as either Latino, Native American and unknown rounded out the study.
"If you look at the whole list, it turns out that whites and blacks are pretty proportionate to their population, very close," said Dave Cullen, author of the book "Columbine," which tells the story of the 1999 massacre at Columbine High School in Littleton, Colorado. Harris and Klebold, the shooters there, were white.
Historically, Latinos and Asians have been the exception.
The Virginia Tech massacre
was carried out by Seung-Hui Cho, who was born in South Korea.
"Latinos are almost nowhere to be seen," Cullen told CNN's "New Day." "Asians continue to be heavily overrepresented -- more than 2½ times their size in the population."
Whites make up about 63% of the U.S. population, blacks 13%, and Asians 5%, according to the latest census numbers. Latinos account for some 17% of the total population.
Much more than race, gender appears to have a direct correlation on whether or not someone becomes a killer.
"Murder is a man's crime," said James Alan Fox, a criminologist at Northeastern University.
Fox, who disagrees with the criteria Mother Jones used to report on mass shootings
, offered various explanations: Men are often more likely to own a gun, to be trained and comfortable around firearms. They often have poorer support systems, and are less likely than women to share their feelings.
Men are responsible for some 90% of all murders. In fact, the only two women included in the Mother Jones study were Jennifer San Marco and Cherie Lash Rhoades.
San Marco, a former postal worker, killed seven people in 2006, then herself. Rhoades opened fire
at a Native American tribal office in Alturas, California, in February 2014. When she ran out of ammunition she grabbed a butcher knife and attacked another person.
"Women tend to see violence as a last resort, as a self-defense mechanism. You use violence if you have to, if there's no other way out," Fox said. "Men tend to use violence as an offensive weapon, to show them who's boss."
Grant Duwe, director of research and evaluation at the Minnesota Department of Corrections, compiled his own numbers for mass public shootings, finding 160 cases between 1915-2013.
Of those, 97 involved shooters who had either been diagnosed with a serious mental illness, or showed signs of one.
"The 61% is actually a minimum estimate," said Duwe, who is also author of the book "Mass Murder in the United States: A History."
"The further back we go in time, we just don't have information as to whether or not they had a mental illness, though in all likelihood, based on all the evidence we have, they most likely did," he said.
Duwe found the most common illness associated with mass public shootings was paranoid schizophrenia, a type of schizophrenia in which the person has delusions of being plotted against or persecuted.
In recent memory, Loughner -- the man who killed six people and wounded former U.S. Rep. Gabrielle Giffords in Arizona -- was diagnosed with paranoid schizophrenia.
He is serving a life sentence
without the possibility of parole after pleading guilty last year to 19 charges in exchange for the government not seeking the death penalty.
Holmes, who was convicted July 16 of killing 12 people at the movie theater in Colorado, has been diagnosed with schizotypal personality disorder, his court-appointed psychiatrist said in testimony. Holmes has pleaded not guilty by reason of insanity.
John Russell Houser, who police say shot 11 people -- killing two -- in a Louisiana movie theater on July 24 before killing himself, was treated for mental health issues in 2008 and 2009
, according to the sheriff of Russell County, Alabama.
Duwe was careful to differentiate between correlation and causation.
"This is not to say that someone that has a serious mental illness is going to go commit a mass public shooting, but what it does suggest is that it is a risk factor," he said.