The suicide rate among U.S. middle school students doubled from 2007 to 2014, surpassing for the first time the incidence of youngsters ages 10 to 14 who died in car crashes, a federal report released on Thursday said.

The rise in middle school suicides, from an annual rate of 0.9 per 100,000 to 2.1 per 100,000, came as traffic deaths in that age group declined to 1.9 per 100,000, according to the Centers for Disease Control and Prevention.

The motor vehicle mortality rate for 2014 marked a 60 percent decline from 1999, when the government began tracking such figures.

In all, 425 young people ages 10 to 14 took their own lives in 2014, compared with 384 who perished in automobile accidents that year, according to the CDC.

Those figures contrasted sharply with figures from 1999, when the rate of middle school students killed in car crashes was four times higher than the rate of those who died of suicide.

“Any rise [in youth suicides] should be of concern, there’s no doubt,” said Mark Kaplan, a UCLA professor of social welfare.

“In time. we might uncover some reasons, but a cautionary note [is] not to rush to any conclusions from this,” Kaplan said.

The underlying causes of suicide are highly complex, making it difficult to explain the trends documented by the CDC, he added.

The leading overall cause of death for Americans 10 to 14 remains accidents of all kinds, including car crashes, accounting for 750 fatalities in that age group in 2014, according to the CDC.