They say that Mary Surratt wept as she heard the gallows that would end her life being constructed outside her prison cell.

A natural enough reaction. Rare is the person who knows the exact time and manner of his or her death. Perhaps she believed she was innocent (doubtful). Perhaps she was bitter that she hadn’t received a fair trial (likely). Whatever the reason, the convicted conspirator in the assassination of Abraham Lincoln couldn’t stop her tears. Some say she cries them still in the building where she was sentenced to hang.

That three-story building is on the grounds of Fort McNair, about two miles southwest of the U.S. Capitol, the gleaming dome of which is visible through the upper windows. Like Mary Surratt and most of the other conspirators, the building was sentenced to death. Unlike them, it survived, though that wasn’t always a sure thing.

“I can’t imagine a worse thing than seeing a building like this destroyed,” said Hans Binnendijk as he led me and some of his colleagues through what was until recently known as Building 20.

Hans is vice president for research and applied learning at National Defense University, the main tenant at the military installation at the confluence of the Potomac and Anacostia. We were on the top floor. Signs of renovation were everywhere: exposed beams, hanging wires, plaster dust. But 146 years ago. . . .

“Right over here is where the defendants sat,” said Hans, pointing toward one end of the room. Spectators lucky enough to snag a ticket to the trial would have been at the other end. Between them were the military judges and members of the press. And in the center: the witness stand.

The producers of “The Conspirator,” Robert Redford’s recent movie about the Surratt trial, came to take measurements of the room. They were fortunate the room was still there to measure.

Designed in 1826 by Charles Bulfinch, the building was originally part of a federal penitentiary. The third floor was a laundry.

“It was the only area in the complex large enough to hold a trial,” said Hans.

Over the years, the other parts of the penitentiary were torn down, and only Building 20 remained. It was threatened several times with demolition. (Stanford White, who designed the neoclassical Roosevelt Hall nearby, hated it.)

“People knew it was there, but no one really cared about it,” Hans said. But Col. Owen Powell, commander of the fort in the late 1990s, recognized its historic importance and ordered it saved. Now, the U.S. Army Corps of Engineers is overseeing a $4 million renovation.

The building will house offices, but the top floor will be returned to how it looked during the trial, complete with furniture used in Redford’s movie. (The movie’s exterior scenes were actually filmed at Fort Pulaski, in Savannah, Ga.) Though it’s on a military base, Hans hopes the public can visit when the renovation is complete, in November.

So the building has a new life — and a new name. It’s been redubbed Grant Hall. “It’s harder to knock down Grant Hall than Building 20,” Hans said.

It has an old life, too: Ghost stories abound, said Susan Lemke, NDU’s special collections librarian. “Presumably she wept near a window,” Susan said of Mary Surratt. “There’s a window there that never dries out.” In hot weather, it’s always fogged. In cold weather, always frosted. Snow melts on a path outside, like footsteps to the gallows. (There’s a tennis court now where Surratt swung.)

Footfalls have been heard in empty rooms as well as the sound of furniture being dragged across a wooden floor.

Hans is dismissive. “The fact is, it’s an old building,” he said. “It creaks.”

I scanned the room, trying to imagine it as it was in the summer of 1865, a president recently martyred, a nation on edge, a trial underway, a woman weeping.

“This was a place where, in some ways, the Civil War ended,” Hans said.

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