In the '80s and '90s, America's suicide trend was headed in the right direction: down.

"It had been decreasing almost steadily since 1986, and then what happened is there was a turnaround," says Sally Curtin, a statistician with the National Center for Health Statistics, part of the Centers for Disease Control and Prevention.

The suicide rate has risen by a quarter, to 13 per 100,000 people in 2014 from 10.5 in 1999, according to an analysis by Curtin and her colleagues that was released Friday.

She says it's heartbreaking to work with these data. While other causes of death are on the decline, suicide just keeps climbing — and it's doing so for every age group under 75.

"I've been losing sleep over this, quite honestly," says Curtin. "You can't just say it's confined to one age group or another for males and females. Truly at all ages people are at risk for this, and our youngest have some of the highest percent increases."

There is one age group that really stands out — girls between the ages of 10 and 14. Though they make up a very small portion of the total suicides, the rate in that group jumped the most — it experienced the largest percent increase, tripling over 15 years from 0.5 to 1.7 per 100,000 people.

And, Curtin points out, in any given year, there are a lot more suicide attempts than there are suicide deaths. "The deaths are but the tip of the iceberg," she says.

Until the suicide trend reversed upward, there had been a number of improvements in the past few decades.

In the late '80s, things were probably looking up partially due to new antidepressants that were more effective and had fewer side effects, says Dr. Maria Oquendo, a psychiatry professor at Columbia University Medical Center and president-elect of the American Psychiatric Association. "We saw this very encouraging decrease in suicide deaths," she says, and the parallel between antidepressant prescription and a decline in suicide was mirrored in other countries. "It was really very remarkable, and somehow that trend toward decreasing suicide rates abruptly stopped in 1999."

What changed? One possibility is economic stagnation, which left more people out of jobs and probably made it harder for people to access health care and treatment. There was also a switch from the use of cocaine and crack to use of heroin and prescription painkillers, which can be lethal in case of an overdose.

And there's also the matter of health insurance — a lot of people weren't covered or didn't have access to treatment for depression, the most common risk factor for suicide. (Since 2014, however, the Affordable Care Act has led to a substantial increase in insurance coverage.)

"Now, the other thing that we were anticipating with some dread was the aftermath of the black box on antidepressants," says Oquendo, referring to a warning label that in 2004 the Food and Drug Administration required for commonly prescribed antidepressants.

The label says that in people under age 26, the medications can actually increase the risk of suicidal thoughts and actions. Research has suggested that the warning scared doctors away from prescribing antidepressants to people of all ages.

"And some of the increment in suicide deaths in the younger populations is potentially linked to an understandable reluctance by physicians who see these youngsters to prescribe antidepressants, even when they're aware that the individual is suffering from depression," says Oquendo. Research has shown that the benefits of prescribing antidepressants to mentally ill children tend to outweigh the risk of suicidal tendencies.

But why such a sharp rise among adolescents, particularly girls? "We don't know what's going on, to be quite honest," says Arielle Sheftall, who works at the Center for Suicide Prevention and Research at the Research Institute at Nationwide Children's Hospital in Columbus, Ohio. "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."

She and her colleagues study the risk factors that might push a depressed child or teen to attempt suicide. One hypothesis about what's going on with girls is pretty surprising: earlier puberty.

"It's usually been referred to as the storm-and-stress period of life because there's just a lot of change happening all at one time," says Sheftall.

Boys tend to hit peak puberty around 13 years old, and girls around 11 years old, though some studies show that girls are starting their periods earlier.

"Research has shown that puberty, unfortunately, is associated with the onset of psychological disorders, specifically depression," says Sheftall.

And depression is a big risk factor for suicidal thoughts and actions. So, because of the shifting age of puberty onset, girls might be opening the door to anxiety, depression and other psychiatric disorders earlier on in life.

Sheftall and Oquendo say the hypothesis hasn't been carefully studied, but it's possible. Another potential factor is that girls attempting suicide could be using more lethal methods, resulting in more deaths.

"It's frustrating because you want to never ever see these trends increase," says Sheftall. "That's what we kind of have dedicated our lives and research to: What is causing these increases to occur?"

For now, there are still more questions than answers.

Copyright 2016 NPR. To see more, visit http://www.npr.org/.