The day after Dr. Martin Luther King Jr. was killed in 1968, Ben Carson’s black classmates unleashed their anger and grief on white students who were a minority at Detroit’s Southwestern High.

Mr. Carson, then a junior with a key to a biology lab where he worked part time, told The Wall Street Journal last month that he protected a few white students from the attacks by hiding them there.

It is a dramatic account of courage and kindness, and it couldn’t be confirmed in interviews with a half-dozen of Mr. Carson’s classmates and his high school physics teacher. The students all remembered the riot. None recalled hearing about white students hiding in the biology lab, and Mr. Carson couldn’t remember any names of those he sheltered.

“It may have happened, but I didn’t see it myself or hear about it,” said Gregory Vartanian, a white classmate of Mr. Carson’s who served in the ROTC with Mr. Carson and is now a retired U.S. Marshal.