"Tonight," LaRoche said, "let every burst of fireworks that illuminates the night sky penetrate the dark recesses of injustice."
While historians knew that Washington owned slaves in Philadelphia, only in the last few years has physical evidence been found, including remnants of an underground passage that his nine slaves used to move about.
That knowledge permeated every facet of the city's July Fourth opening ceremony - the invocation, the benediction, Mayor Street's speech, a scene from a play about the slaves and LaRoche's remarks.
Washington lived in the house, a half-block from where the Declaration of Independence was signed, in the 1790s.
LaRoche said he knew of the slaves' quest for freedom. Four of Washington's own planned or attempted escape at some point during or after their captivity in Philadelphia.
Just two - Oney Judge, who attended Martha Washington, and Hercules, the chef - succeeded.
"They seized the freedom the Declaration of Independence promised but the nation did not deliver," LaRoche said.
Street, in his remarks, said the history of slavery "is a story with deep roots in Philadelphia."
He said the city had been a leader in the abolitionist movement. Today, he said, it "is playing a leading role in acknowledging a blight on our history, by telling the complete story of the President's House."
Just behind the audience he was addressing rose a mound of dirt, covered by a blue tarp, from the dig.
"Slavery and the presidency, the Declaration of Independence and human bondage, all existing side by side, right here where you are sitting today," Street marveled.
As recently as May, officials planned to finish the dig and fill in the hole by yesterday's ceremonies. When they opened the site to the public and more than 1,000 people visited in one weekend, they reconsidered.
Activist Michael Coard and the group he founded, Avenging the Ancestors Coalition, called for a permanent memorial. City and National Park Service officials are pondering how best to do that.
Yesterday, visitors surged to a small platform overlooking the dig, where archaeologists spoke about the work.
By noon, more than 2,200 people had already visited, said historical archaeologist Patrice L. Jeppson. Officials expected 10,000.
Elizabeth Walker of Bethlehem, Pa., spoke of the contradiction represented there, the "dark side" of history juxtaposed against the promise of liberty and freedom.
Her daughter Ruvenia, an African American studies major at Temple University, said she was "shocked, very, very shocked," but also glad history had not been "airbrushed."
Contact staff writer Sandy Bauers at 215-854-5147 or firstname.lastname@example.org