African-Americans are twice as likely as whites to die from sudden cardiac arrest, and they are younger on average when it happens, too, researchers reported Monday.

It’s more troubling evidence that blacks have more severe heart disease than whites, but it’s still not clear why.

Sudden cardiac arrest can be caused by heart attacks but it’s also caused by irregular heart beat and electrical disturbances. The study of more than 100 blacks and 1,200 whites who suffered cardiac arrest in the Portland area showed that blacks were, on average, six years younger than whites when it happened.

Blacks were also more likely to have other diseases, such as diabetes, high blood pressure and kidney disease, than whites.

“We do not know why African-Americans are more likely to have sudden cardiac arrest,” said Kyndaron Reinier of the the Cedars-Sinai Heart Institute, who led the study.

“It could be due to the higher burden of illnesses that increase risk of heart disease, like hypertension and diabetes. Or it could be genetic because we know that certain health conditions are more prevalent in particular groups of people. Or, the reason could be environmental, such as access to good healthcare. But there is no doubt that there are differences between the races when it comes to clinical outcomes.”

The study, published in the journal Circulation, adds to a growing number of studies showing that U.S. blacks are far more likely than whites to suffer from many chronic diseases, from heart disease to some types of cancer. Heart attack rates are higher among African-Americans, for instance.

And there’s some evidence that it’s not all due to different diets, different lifestyles or even different access to health care. There’s evidence of biological differences, such as the discovery that one test of heart disease risk is more accurate in black women.

“Because sudden cardiac arrest is usually fatal, we have to prevent it before it strikes,” said Dr. Sumeet Chugh of Cedars-Sinai, senior author on the study. “These findings suggest the possibility that when it comes to prevention of sudden cardiac death, different races and ethnicities may not necessarily be painted with one broad brush.”