CHICAGO (AP) — Suicides by young children are rare and the low rate has held mostly stable for two decades except for a troubling increase among black boys.

A new study found that from 1993 through 2012, there were 657 suicides among children aged 5 through 11. Most were 10 or 11.

The overall rate for young kids was just over 1 per million. But for black boys, it went from less than 2 per million to almost 3.5 per million. There was a slight decline among white boys, to 1.3 per million.

Lead author Jeffrey Bridge says reasons for the disparities are uncertain. He’s a suicide researcher at the Research Institute of Nationwide Children’s Hospital in Columbus, Ohio.

The study was published online Monday in JAMA Pediatrics.