Lay researchers have long played an outsize role in Lincoln scholarship, especially when it comes to the assassination. The historian Mark E. Neely Jr., writing in 1979, lamented that scholars had largely ignored the topic, leaving the field open to “an avalanche of absurd sensationalizing” and conspiracy theories.

That scholarly reticence has lifted somewhat in recent years as studies of how the Civil War is remembered, exemplified by books like C. Wyatt Evans’s “The Legend of John Wilkes Booth” (2004) and Martha Hodes’s “Mourning Lincoln,” published in February, have gained ground in the academy. Still, Mr. Holzer summed up the lingering aversion to spending too much time focusing on the assassin.

“Booth is repulsive and a murderer,” he said. “This was a man who, with a single act, may have postponed for 100 years the chance of any kind of racial reconciliation.”

Into the void have swept the Boothies, a loose network of people who congregate at sites like Tudor Hall, the Booth family home near Bel Air, Md., and the Surratt House Museum in Clinton, Md., the former family home of a convicted co-conspirator. Online, the Boothies gather in places like the Lincoln Discussion Symposium, which has more posts on the assassination than on nearly all other topics combined.

The Surratt house, with its costumed guides, attracts some 10,000 visitors a year, and nearly 150 researchers from around the world attended its annual conference this year, including a few mainstream Lincoln scholars. Its 12-hour bus tour of Booth’s escape route through Maryland and Virginia regularly sells out, as does a similar one sponsored by Smithsonian Associates.

Historians credit Boothies with valuable research on topics like Booth’s weaponry, the flags that hung in Lincoln’s box at Ford’s Theater and even the Pennsylvania oil leases Booth speculated in.

Image John Wilkes Booth Credit... Library of Congress

“They are familiar with the most arcane details,” said Thomas A. Bogar, a retired professor at Hood College in Frederick, Md., and the author of “Backstage at the Lincoln Assassination,” a study of the 46 Ford’s Theater employees who were swept up in the investigation. “It stops short of that magic line between extreme interest and obsession.”