Earlier this year, researcher Rebecca Siegel of the American Cancer Society published a startling report showing that colon and rectal cancer incidence is rising among Gen X and millennials while falling in older generations.

On Tuesday, after delving further into the data, she and her co-authors identified “a true and perplexing escalation in disease occurrence.” In a paper in JAMA, the Journal of the American Medical Association, they report that deaths from colorectal cancer are increasing for young and middle-aged Americans — though the increase appears, at least so far, to be confined to white men and women.

The U.S. mortality rate for colorectal cancer for ages 20 to 54 fell from 6.3 per 100,000 in 1970 to 3.9 in 2004. It then began to go up by 1 percent annually. By 2014, the rate was 4.3 per 100,000.