New federal data released Tuesday found the number of new diabetes diagnoses fell to about 1.3 million in 2017, down from 1.7 million in 2009.

Earlier research had spotted a decline, and the new report shows it's been going on for close to a decade. But health officials are not celebrating.

"The bottom line is we don't know for sure what's driving these trends," said the lead author of the new report, Dr. Stephen Benoit of the Centers for Disease Control and Prevention. Among the possibilities: Changes in testing and getting people to improve their health before becoming diabetic.

The report was published by the journal BMJ Open Diabetes Research & Care. The statistics run through 2017. Last year's numbers are not yet available, Benoit said.

Diabetes is a disease in which sugar builds up in the blood. The most common form is tied to obesity, and the number of diabetics ballooned as U.S. obesity rates increased.

But other factors also might have pushed up annual diabetes diagnoses from 2000 to 2010, and they may partly explain why the numbers have been going down since, some experts said.