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A sign for an emergency phone is seen on the span of the Golden Gate Bridge in 2008 in San Francisco. Getty Images

The suicide rate increased for Americans of both sexes and nearly all age groups between 1999 and 2014, but there's one contingent that's been hit particularly hard: young teen girls. A study released Friday by the National Center for Health Statistics at the Centers for Disease Control and Prevention found that the suicide rate for girls between 10 and 14 increased by 200 percent.

"We don't know what's going on, to be quite honest," Arielle Sheftall, a postdoctoral research fellow at Nationwide Children's Hospital in Columbus, Ohio, told NPR. "We have thoughts that maybe it's this, maybe it's that. It's really hard to pinpoint one specific risk factor that really, truly is driving this trend."

The CDC found that in 1999, the suicide rate for young teen girls was 0.5 per 100,000 people. In 2014, it was 1.5. That means that though the rate is still relatively low — the suicide rate for women between 45 and 64, for example, was 9.8 — experts are concerned because it's risen so quickly.

Sheftall suggested to NPR that early puberty could be behind the rash of teen girls taking their own lives. Some mental illnesses, like depression, are linked to the beginning of puberty, which occurs on average earlier for girls than boys.

Girls consider suicide and try to kill themselves about twice as often as boys do, according to kidshealth.org. But boys have historically been more successful, in part because they tend to try more violent methods. The CDC data showed the suicide rate for boys 10-14 jumped from 1.9 to 2.6 per 100,000 between 1999 and 2014.

But girls' self-harm shouldn't be overlooked, according to Sally Curtin, a CDC statistician who worked on the report. “For that group, the deaths are just the tip of the iceberg,” she told PBS. “There are so many more attempts and hospitalizations.”

Teen suicide attempts have been in the news recently due to a state of emergency in Attawapiskat, Ontario, Canada. The First Nation native community, which has a population of about 2,000 people, saw nearly 30 people try to kill themselves last month, the National Post reported. This past Monday alone, 12 teenagers tried to take their own lives.

If you are in crisis or know someone who might be, Curtin recommended calling the National Suicide Prevention Lifeline at 1-800-273-8255.