Gerald S. Padmore, board member of the Northern Virginia Urban League, and Cynthia M. Dinkins, president and chief executive of the group, examine the Freedom House Museum. (Patricia A. Sullivan/The Washington Post)

Visitors to the Freedom House Museum in Alexandria often say they get shivers when they descend the stairs to the basement.

The small, brick-walled cellar of this 1812 rowhouse, with its original iron bars and replicas of shackles and a whip, is an artifact of the chilling story of the thousands of enslaved people who passed through here between 1828 and 1861.

This was the largest slave-trading depot in the nation, where about 1,800 people were held each year before being sent to the plantations and fields where many toiled until they died. It’s thought that Solomon Northup, whose life story was told in the 2013 movie “12 Years a Slave,” might have passed through here. Cholera swept through the group of detainees, at one point killing “nine Negroes and six or seven children. Seven or eight Negroes are sick,” according to a property owner’s diary.

All the while, the white slave traders lived and worked on the floors above them.

The building at 1315 Duke St. is an irreplaceable historic site. So when a pipe burst and the sprinklers turned on Feb. 16 during the winter storm that walloped the Washington area, the stakes were extraordinarily high.

Cynthia M. Dinkins, the president and chief executive of the Northern Virginia Urban League, a nonprofit social services organization that now owns the property, was relieved that the museum proper was spared. Some of the league’s offices, however, suffered major damage. Ceilings collapsed, carpets were sodden and ruined, computers shorted out and many books and papers were lost. The space was used for programs for young mothers, grandfathers and young professionals, as well as offices.

“My heart hurts,” Dinkins said as she walked through the offices. “Coming in here, you feel so empowered about how far we’ve come. Now,” she gestured to the stacked boxes and walls stripped down to the studs.

“Any step back is a giant leap back,” agreed Gerald S. Padmore, an executive board member. “But no matter what, the Urban League has to continue to thrive.”

Dinkins and the board members are still assessing the structural damage for their insurance company. They think it will exceed $100,000. The museum and offices were forced to close for the remainder of Black History Month. The building cannot reopen until repairs are made and the water is turned back on.

The Northern Virginia Urban League bought the building in 1996 and has been working ever since at developing it into a museum that documents the history of the slave trade. The basement has multimedia exhibits, touchable artifacts and historical timelines. The league and its partners are working on a teaching curriculum about slavery.

Isaac Franklin and John Armfield leased the building in 1828, later buying it outright. The men used it and its grounds as holding pens for Africans and African Americans brought to Virginia through the Alexandria port. They bought the people from bondsmen and shipped them to Natchez, Miss., and New Orleans, where the slaves were sold again. It was a profitable business and well-known as both the “Franklin and Armfield Slave Office” and “the Alexandria slave pen.”

Slave traders owned and operated the property until 1861, when Union troops seized it at the start of the Civil War and turned it into a military prison for Confederates.

“This building is a jewel not just for Alexandria, not for Virginia but for the country and world,” Dinkins said. “This building was the epicenter for the slave trade and it’s wonderful karma that it’s now owned by the Northern Virginia Urban League. . . . We hope that people, preservationists, neighbors and friends will partner with us to preserve this national historical landmark and treasure.”