“There is no better site,” Mayor Joseph P. Riley Jr. said Tuesday, standing on the waterfront tract where the 42,000-square-foot museum will be built in the city where the Civil War began. It’s near where a wharf where slaves left ships once stood.

The site is just down and across the street from the vacant lot where the museum, first proposed 13 years ago, was originally planned.

Riley said that as research for the museum was done, the significance of Gadsden’s Wharf became evident. The wharf was built by Revolutionary War patriot Christopher Gadsden and it’s estimated that 40 percent of African slaves brought to the United States in the late 18th and early 19th centuries walked across it.

From 1803 to 1807, the final years of the international slave trade, more than 70,000 enslaved Africans were brought to the wharf at a time when Charleston’s population was only 20,000. The first slaves arrived in Charleston in 1670, the same year the Carolina colony was founded.

Riley said that in recent months, during discussions with museum architects, it was determined that, if possible, the museum should be built at the site.

Part of the tract is city land. A portion had been sold to a family that is planning to build a restaurant. But the mayor said they agreed to sell the land back to the city as a site for the museum.

“We always knew this museum would be one of the most important in our country,” the mayor said. “We are standing on the site that will make this museum even more powerful and important and will resonate more deeply with everyone who attends.”

“The discovery of this site adds many dimensions to the telling of that story. It adds an historical integrity to that story,” added Wilbur Johnson, a local attorney and chairman of the museum board.

Ralph Appelbaum, who designed the exhibits for the Holocaust Memorial Museum in Washington and the new Visitor Reception Center at the United States Capitol, is designing the exhibits for the museum. The museum will tell the story of black Americans with the use of interactive displays and changing exhibits.

Plans call for one third of the money to come from private donations, one third from Charleston city and county and one third from the state.

Riley said that he hopes the money will be in place by early 2016 so construction can begin. If that schedule holds, the museum should open two years later.