The apparent manifesto of the alleged Charleston, South Carolina, gunman surfaced online Saturday and paints the picture of a white supremacist radicalized online, experts said, with the text reading like its author soaked up ideologies and was spurred to extreme action -- an uncommon but deadly occurrence.

“Lone wolf killers are still relatively rare,” said Mark Pitcavage, director of the Anti-Defamation League's Center on Extremism. “In terms of people who actually kill, they’re relatively rare.”

Dylann Roof, 21, is the suspect charged in the fatal shootings Wednesday of nine people inside a historically black Emanuel African Methodist Episcopal church.

Pitcavage said he tracked lone wolf shooters for nearly two decades beginning in 1993 and found 35 cases. Those who act out as Roof allegedly did are relatively uncommon but exceptionally dangerous. The manifesto that surfaced seemed to suggest its writer fits the profile: a person who builds anger but remains isolated and doesn't necessarily reach out to the groups from which they borrow ideas.

"It is entirely compatible with the lone wolf paradigm of someone who imbibes the ideas of an extremist movement but does not ever become truly active in it,” Pitcavage said. "It’s a phenomenon we're all too familiar with."

The manifesto (Warning: Very offensive content), which is about 2,400 words, references views on segregation and slavery, and is broken into sections detailing the writer's beliefs on different races. The site is believed to be legitimate, but authorities are still working to confirm its authenticity, ABC News reported.

Pitcavage pointed to a specific reference to the Council of Conservative Citizens website in the manifesto that was particularly telling, citing the organization as a group that descended from those who opposed desegregation. 

The first website I came to was the Council of Conservative Citizens. There were pages upon pages of these brutal black on White murders. I was in disbelief. At this moment I realized that something was very wrong. How could the news be blowing up the Trayvon Martin case while hundreds of these black on White murders got ignored?

The manifesto also references the idea of the "Northwest Front," a term coined by the neo-Nazi Harold Covington who has been active in white supremacy since the 1970s. It's the longstanding idea that whites in the United States should move to the Pacific Northwest and start a new civilization. Pitcavage said the manifesto purportedly created by Roof reads as though the writer had selectively settled on an ideology based on the information that best suited him. 

"[The writer is] someone who had been recently exposed to white supremacy ideas," he said. "Trying on a few ideas for size, seeing which ones fit."

Heidi Beirich, director of the Southern Poverty Law Center's Intelligence Project, agreed the reference to the Council of Conservative Citizens was of particular concern.

 "All [the council site] talks about it, black-on-white crime, the idea that whites are under assault," Beirich said. "It’s another example ... of someone being radicalized on an extremist website."

Should it ultimately be proven that Roof penned the manifesto, it would connect him to a troubling issue Beirich said she has seen growing over the years. "The problem in general here," she said, is "we have homegrown terrorists that are dropping bodies in the United States."

Experts cited recent similar cases, among them that of Frazier Glenn Miller, accused of killing three people at Jewish centers in Kansas; Keith Luke, who killed two people in a hate-filled attack and later committed suicide in prison; and Wade Michael Page, who killed six people in a Sikh Temple in Wisconsin

While the actions Roof is accused of taking are rare, Beirich said, what was in the manifesto hardly is. "It's just standard white supremacist B.S.," she said.

And while there are likely a fair number of people who have thoughts similar to those in Roof’s purported manifesto, it was the ending of the text that was the most telling. Pitcavage noted after explaining why Charleston was chosen as the place for extreme action, the final line reads: "Please forgive any typos, I didnt have time to check it."

"That’s what he wants to be forgiven for: typos," said Pitcavage, his voice rising. "Not this murderous act ... typos."

But he also pointed out shooting at a church was an exceptionally cowardly act since a church is the "softest target, the most vulnerable target imaginable."