Outside a Georgetown restaurant a group of crime writers is calling it a night when somebody brings up an old case one of them has been mining for years.

“Guilty!” declares James Swanson, author of “Manhunt” and other books about Abraham Lincoln’s assassination. “Guilty! Guilty! Guilty!”

In Swanson’s opinion, one person associated with the plot to kill the 16th president didn’t get the punishment he deserved: Samuel Mudd, the Charles County, Md., physician who treated John Wilkes Booth’s broken leg after the actor shot Lincoln.

Tried by a military commission with other accused conspirators, Mudd escaped being hanged by a single vote. Instead, he was sentenced to life in prison at Fort Jefferson, a 19th-century military outpost in Florida’s Dry Tortugas .

April 14 and 15 will mark the 150th anniversary of Booth’s attack on Lincoln in Ford’s Theatre and the president’s death the following day. Ever since, scholars and amateurs alike have puzzled over Mudd’s role in events surrounding the assassination. I’ve always been interested in historic bit players such as Mudd and after checking my calendar decided to wade in with my own modest research project. Although “research” could be the wrong word for a fact-finding trip that takes me from midwinter Washington to the Gulf of Mexico and what has been called America’s “most exotic national park.”

A travel Web site describes the remote Dry Tortugas as a “stark and stunning” archipelago that attracts thousands of visitors each year. Which might make it hard to imagine the forbidding scene it had to be in Mudd’s time. So if I’m going to follow the trail that got him there, maybe I should start with some modern-day Mudds.

At the Ford’s Theatre Museum, John Wilkes Booth’s gun, used to shoot Lincoln. (Michael S. Williamson/The Washington Post)

Conor McHale, the doctor’s great-great-great-granson, and I are getting together for lunch at Wok and Roll in Chinatown. Miss the plaque on the building, and you’d never know this was once Mary Surratt’s boarding house, where the conspiracy against Lincoln was planned.

McHale, editor of the Dr. Samuel A. Mudd Society newsletter, is well versed in the life of his medical ancestor. Many of his relatives are, too, and a pair is joining us: Mary McHale, Mudd’s great-granddaughter, and Robert Summers, his great-.

They agree theirs is a genealogy with a unique set of baggage.

“When I was in the first grade,” Conor recalls, “the principal found out I was related to Mudd and told the whole class.” He felt “a certain sense of prestige,” he says, at being genetically linked to one of the most famous episodes in U.S. history.

By the time our main course comes, we’re already deep into a discussion of what Mudd knew and when he knew it the morning Booth and accomplice David Herold showed up at his farm six hours after Lincoln was shot.

Mudd, a Confederate sympathizer and slave owner, had met Booth on two occasions in late 1864: once when Booth said he was looking for land to buy near Waldorf, Md., and later in Booth’s Washington hotel room.

Some historians believe those meetings connect Mudd to the murder. Summers, who spent years investigating the evidence for his book “The Assassin’s Doctor,” says, “Mudd had nothing to do with the assassination, which was organized the day it happened.” But, he adds, Mudd “clearly lied” about not knowing Booth. He also waited more than a day before getting word to authorities that Booth and Herold had stayed at his house. “He helped them avoid capture. The commission got it right.”

Of course, that’s not how everyone in the Mudd clan sees it.

This July 24, the anniversary of Mudd’s arrival at Fort Jefferson in 1865, dozens of his descendants will travel to the westernmost Florida Keys to protest the military trial that sent him to prison. Conor McHale says everyone will be wearing “Free Dr. Mudd” T-shirts.

I tell him my research trip can’t wait till July, but I would like a T-shirt.

At the Dr. Samuel A. Mudd House Museum in Waldorf, Md., Conor McHale sits in the back yard of the Mudd home. McHale is the great-great-great-grandson of Mudd. (Michael S. Williamson/The Washington Post)

After exiting the Capital Beltway, I pick up Booth’s escape route at the Surratt House in Clinton. The restored tavern, once operated by Mary Surratt and her husband, John, was the assassin’s first scheduled stop. At some point during the night Booth broke his leg, most likely when his horse took a spill. Experts question his diary account that it happened when he jumped from the president’s theater box to the stage. Whatever the case, it meant a 12-mile detour to see Dr. Mudd.

Mudd’s two-story house, refurbished and open to visitors, is near the eastern end of Zekiah Swamp. This region was notorious for its Confederate leanings. It was also a hangout for smugglers eager to take paying customers across the Potomac into Virginia.

Danny Fluhart, president of the Mudd Society, offers to show me around. Between two parlor windows is the sofa Booth sat on while Mudd examined his injured leg. Upstairs is a replica of the bed where Booth slept, under which Sarah Mudd found the most incriminating piece of evidence against her husband, a boot with “J. Wilkes” inside. After his wife gave it to investigators, Mudd was arrested.

I ask my guide if he thinks Mudd was guilty.

“You can believe whatever you like outside,” replies Fluhart. “But inside this house, he’s innocent.”

In the Dry Tortugas off Florida, visitors approach the old prison where Mudd served out his days. (Michael S. Williamson/The Washington Post)

Shortly after the Civil War began, martial law, the suspension of First Amendment rights and other measures imposed on their state made many Marylanders hostile toward the new president. It’s no coincidence that six of the eight charged with conspiring in Lincoln’s murder were from Maryland. Booth, who was gunned down 12 days after the assassination, was from Bel Air, northeast of Baltimore.

[The Post’s special coverage of the 150th anniversary of the Civil War]

Predictably, military justice was swift. Four of the accused were hanged the day after the verdicts. Three of the other four, including Mudd, were sentenced to life; a stagehand who tended Booth’s horse got six years.

In a newspaper interview, Mudd said, “... not a man of them sat on my trial with an unbiased and unprejudiced mind. Before a word of evidence was heard … I was already condemned. ... ” If that’s true, avoiding the hangman’s noose qualifies as a legal miracle, even though the alternative was nothing to look forward to.

Most of South Florida was a tropical jungle when the four “federal prisoners” arrived by Navy gunboat after a week at sea. Today, the jungle has been replaced by shopping malls, and our trip takes only two hours. My wife, Irina, and I fly into Fort Lauderdale, pick up a rental car and head for lunch in Miami’s Little Havana. Then it’s on to Key West, our port of departure for the Dry Tortugas, 70 miles into the Gulf of Mexico.

Coming from Russia, where assassinations are routine in politics, my wife had no interest in Lincoln’s, but she does like Florida.

In Little Havana the recent proposal to normalize relations with Cuba is not popular. The last time I was here, more than 20 years ago, Frank Sturgis of Watergate break-in fame was training a small army of exiles for a Cuba invasion. I came to interview him and ended up spending a week at his commando base in the Everglades. When I tell Irina about going on maneuvers in alligator-infested swamps, she likes the idea.

“Let’s go see alligators,” she says, scrolling through Everglades attractions on her smartphone.

I want to get started for Key West, but then she finds Captain Mitch’s Airboat Tours , “redneck owned and operated” for more than 60 years.

We take off for gator country.

Captain Mitch’s, just north of Everglades City, looks like a scene in “Creature From the Black Lagoon.” The story goes that Mitch’s father fashioned his first airboat out of plywood and a surplus airplane propeller. In no time, tourists from all over were coming to marvel at alligators.

As the only customers late on a Saturday, we get the boat all to ourselves. “Ever been in the Everglades before?” asks our guide, Captain Rick, as we climb in.

I mention being at Sturgis’s secret commando base, and he remembers it.

“Oh, yeah, I used to see those guys all the time.”

I guess it wasn’t that secret.

Only minutes into our trip, we spot a “medium-size” alligator sunning itself in swamp grass. Captain Rick explains how gators, being cold-blooded, collect heat in “little solar panels” on their backs. “There’s gonna be way bigger ones than this,” he assures us.

With the airboat now in overdrive, we speed across an open channel, then fishtail into a thick stand of mangroves, coming to a stop alongside a seven- or eight-foot gator. Even Captain Rick is impressed.

“These ol’ boys can be very aggressive,” he says. “If they’re looking for fresh water, they’ll go right into people’s kitchens.”

That reminds me of an Elmore Leonard novel. No writer understood the Florida criminal mind like Leonard, and in “Maximum Bob” one of his bad guys uses an alligator as a murder weapon. Entirely possible, says Captain Rick.

After an hour of tooling around in the swamps, it’s time to go, but not before Irina gets a look at the baby alligators back at the office. There are two boy gators, or they could be girls. Apparently there’s no way of telling with babies.

My wife picks one up and starts making little chirping sounds. The baby gator cocks its head to listen. Captain Rick can’t believe it.

“Never seen that before. I think she speaks alligator.”

Swimmers and sunbathers take in the view by Fort Jefferson. (Michael S. Williamson/The Washington Post)

Early the next morning we’re on our way to the Keys. Breezing down Route 1, across Key Largo, I’m enjoying the view until I notice “road prisoners” working along the highway. The prisoners, 20 or 30 of them, are being watched by armed guards. It’s a slight mood killer, but it reminds me why we came.

This vacation is about crime and punishment, I tell Irina.

“With some fun in between, right?” she says.

By sundown, we reach Key West, the final bit of green on our GPS device before everything turns blue. In the past several decades the city has become a mini-New Orleans, where cruise ships alone unload more than 800,000 tourists annually. One appeal is that local officials pay only half attention to what the tourists do. Fortunately, we just missed Fantasy Fest, when people you’d rather see clothed parade around semi-nude.

If Key West wasn’t the inspiration for “Margaritaville,” Jimmy Buffett’s 1977 hit about day drinking, it could have been. Actually, Buffett wrote most of the song in Austin before adding the finishing touches here. Now it’s the town’s musical mission statement and a big reason Key West has gone from the kind of place President Harry Truman visited on vacation to the nation’s capital of hanging out.

But the city, full of quaint cottages and such legendary dives as Sloppy Joe’s, also has a literary side. A popular pre-happy hour activity is visiting the former homes of Ernest Hemingway, who wrote “The Snows of Kilimanjaro” in his house on Whitehead Street, and playwright Tennessee Williams, who lived on Duncan Street and has a community theater named after him.

Another celebrity residence that attracts more adventurous types is Dr. Samuel Mudd’s offshore prison cell at Fort Jefferson.

When Irina and I get to the ferry terminal, the skies are overcast, and morning weather reports are calling for seven-foot waves. “That’s nothing,” says a park ranger who works at the fort. “Once the waves were so high we had to turn back.”

I pop a couple of Dramamines right before we shove off. My wife prefers to take her chances. The ferry staff has prepared a breakfast buffet, and, thanks to the meds, I’m on my second ham sandwich when the crew begins handing out seasickness bags.

Fort Jefferson was thought to be escape-proof. Mudd tried once and failed. (Michael S. Williamson/The Washington Post)

After a rough ride, which we both survive without incident, Fort Jefferson comes into view. Calling the Dry Tortugas “islands” may be an exaggeration. There’s even uncertainty about how many Tortugas there are, since a number disappear and reappear on a regular basis. Sometimes there are seven; other times up to 11.

Construction of the six-sided brick fort began in 1846 and was still going on when the prisoners from Washington landed. By then, the population of just under 1,000 included almost equal numbers of federal troops and Union deserters sentenced to hard labor. Our ferry docks, and suddenly we’re on dry land in the middle of the Gulf. It’s impossible not to think of how it must have felt for Mudd to realize this is where he’d spend the rest of his life.

Irina packs to go snorkeling. I start exploring. Armed with a map, I climb the stairs to Mudd’s cell, an unfinished gun room above the main entrance. At first I think this could be nice, if it didn’t come with a life sentence. Still, it’s airy and bright with a great view. Three vertical slits serve as windows overlooking a vast expanse of blue-green sea.

Fort Jefferson was thought to be escape-proof. Just the same, Mudd made a try after only two months by hiding on a supply ship. His great-grandson Robert Summers believes his ancestor “was afraid of how he would be treated” by the 82nd U.S. Colored Infantry, newly assigned to guard duty.

Following a failed escape, Mudd lost his job in the fort’s hospital but was pressed back into service two years later when the resident doctor died of yellow fever. Many guards and prisoners also perished. Despite coming down with the disease, Mudd went on to save dozens of lives. His efforts were so heroic that soldiers petitioned President Andrew Johnson to pardon him. Which he did a month before leaving office in 1869.

It would make a great movie. Twentieth Century Fox must have been thinking the same thing in 1936 when it released “The Prisoner of Shark Island,” directed by John Ford. The film, much of it fiction, portrays Mudd as an innocent victim of circumstances.

The park ranger I met at the ferry terminal says he’s heard that one of Mudd’s relatives works at the fort. At park headquarters the ranger in charge locates Allen Zamrok, who says he’ll be right over. Zamrok, “Rock” to friends, grew up in La Plata, Md., and is distantly related to Mudd. His grandmother’s cousin married one of Mudd’s sons.

Rock agrees it’s pretty strange to be working where he is. His job involves desalinating seawater to make it drinkable. They call these islands the Dry Tortugas because there’s no fresh water.

He says he has never been that concerned about Mudd’s legal problems. But spending a few nights a week at Fort Jefferson has made him wonder.

“It’s really peaceful after the boat goes back to Key West,” he says. “I guess if you could never leave, it would be a lot different.”

At Ford’s Theatre, volunteer Mike Robinson (in character as a police investigator) points out where the president was sitting when he was shot. (Michael S. Williamson/The Washington Post)

Not long after returning to Washington, I drive to Bryantown, Md., in Charles County. Mudd is buried here with his wife in the cemetery beside St. Mary’s Catholic Church. This is where he was introduced to John Wilkes Booth before Sunday Mass in November 1864.

Friends welcomed Mudd home without fanfare, says historian Michael Kauffman, author of “American Brutus: John Wilkes Booth and the Lincoln Conspiracies.” “But I think in their own quiet way, they identified with him, even regarded him as a martyr.”

Kauffman finds it interesting that Mudd’s grave is located so prominently in the front of the cemetery. “It’s as if people wanted his tombstone to be a reminder,” he says, “not only of one man, but of the role their little village played in the nation’s history.”

Bill Thomas, author, journalist and founder of Private Toursof Washington, is a frequent contributor to the Magazine. To comment on this story, e-mail wpmagazine@washpost.com or visit washingtonpost.com/magazine.

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