The U.S. Capitol today added a physical acknowledgment of the role slave labor played in the construction of the building.

Leaders from both parties and both houses of Congress unveiled a commemorative marker in the Capitol Visitors Center to pay tribute to those who put the building up.

The marker features a single block of sandstone, once part of the Capitol's East Front portico, placed in reverse position so that the original chisel marks, done by those who built the building, are clearly visible.

The construction of the Capitol relied heavily on slave labor. Slaves performed the backbreaking work of quarrying the stones found in the Capitol's floors, walls and columns like the one that today turned into a commemorative piece.

"Through the unveiling of this marker today, we finally permit countless and nameless souls to rest," Rep. John Lewis said at the ceremony this afternoon, "We honor the work, the dedication, the artistry, the imagination, and the contribution of men and women in chains who help us, even at this hour, to sanctify the U.S. Capitol as our 'Temple of Liberty.'"

"The history of the capitol, like the history of our nation should be complete," Senate Minority Leader Mitch McConnell, R-Ky., said, "because we have always aspired to something better that work may never be complete. But as long as we remain true to our purpose as a nation, liberty and dedicated to the provocation that all men are created equal, we will continue as it must."

Rep. Nancy Pelosi, D-Calif., said that the marker is a memorial to the "tragedy and sin" of slavery.

"For too long, the sacrifice of men and women who built this temple of democracy were overlooked; their toil forgotten; their story ignored or denied, and their voices silenced in the pages of history," Pelosi said. "Yet today, we join together to strive to right this wrong of our past, to honor the sacrifice of these laborers, to lay down a marker of gratitude and respect for those who built the walls of the Capitol."

The bicameral, bipartisan leadership collectively pulled a rope to unveil the large new marker.

"These laborers went unrecognized for generations," Speaker of the House John Boehner, R-Ohio, said, "and all those honored by this marker have done our Capitol and our country a great service."