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.

Breaking the numbers out by race reveals the  trend, with the outlook becoming better for blacks and people of other races but not for whites.

The mortality rate for whites had been declining for decades, but it began to climb starting in 2004, going from 3.6 per 100,000 to 4.1 per 100,000 in 2014. For blacks, it went down to 6.1 per 100,000 in 2014. While that's still higher than for whites, the trend is heading in the right direction. For other races, mortality rates declined until 2006 and remained stable through 2014.

Siegel noted that the racial patterns are “inconsistent” with trends in major risk factors for colorectal cancer. These include obesity, which is increasing among all races and would suggest colorectal cancer rates should be going up among all races, too.

The data, which encompass almost  250,000 people, come from the National Center for Health Statistics and information it collects from death certificates. They raise more questions on whether the country's screening standards should be revisited. Current guidelines call for regular screening for colorectal cancer to begin at age 50.

The analysis of mortality rates contained another surprise: They are also rising for white men and women in their 50s despite the actual cancer incidence decreasing and screening recommended for this age group for decades. In contrast, the mortality rates for blacks in their 50s has declined since 1993.

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