President Obama’s biography — son of a black father from Kenya and a white mother from Kansas — has long suggested that unlike most African-Americans, his roots did not include slavery.

Now a team of genealogists is upending that thinking, saying that Mr. Obama’s mother had, in addition to her European ancestors, at least one African forebear and that the president is most likely descended from one of the first documented African slaves in the United States.

The findings are scheduled to be announced on Monday by Ancestry.com, a genealogy company based in Provo, Utah. Its team, while lacking definitive proof, said it had evidence that “strongly suggests” Mr. Obama’s family tree — on his mother’s side — stretches back nearly four centuries to a slave in colonial Virginia named John Punch.

In 1640, Mr. Punch, then an indentured servant, escaped from Virginia and went to Maryland. He was captured there and, along with two white servants who had also escaped, was put on trial. His punishment — servitude for life — was harsher than what the white servants received, and it has led some historians to regard him as the first African to be legally sanctioned as a slave, years before Virginia adopted laws allowing slavery.