WASHINGTON — When Michelle Obama said in her prime-time televised address to the Democratic National Convention on Monday night that the White House had been built by slaves, she was citing a little-discussed fact that dramatized her own African-American family’s place in history.

But the first lady’s assertion was met with derision and disbelief by some, who questioned whether it was true and said her choice to mention it amounted to an attempt to divide the country along racial lines.

There is little dispute among historians that slaves had a role in the building of the White House. According to the White House Historical Association’s website, planners had initially intended to import workers from Europe but had trouble recruiting any, so they “turned to African-American — enslaved and free — to provide the bulk of labor that built the White House, the United States Capitol, and other early government buildings.”