WEDNESDAY, April 25, 2018 (HealthDay News) -- As the United States grapples with an ongoing opioid epidemic, experts are calling attention to a hidden aspect of the crisis: Many overdose deaths may, in fact, be suicides.

The researchers describe suicide as a "silent contributor" to the nation's opioid overdose death rate.

It's hard to know exactly how many Americans have intentionally overdosed on opioids in recent years, said report author Dr. Maria Oquendo, a professor of psychiatry at the University of Pennsylvania. The analysis of the issue is published April 26 in the New England Journal of Medicine.

One problem, she explained, is that there are different ways of establishing a "manner" of death across the country. "Manner" refers not to the cause -- a drug overdose, for example -- but whether a death was a homicide, suicide or accident.

Unless there's a suicide note, or documented history of depression, it may be impossible to establish a drug overdose as a suicide.

In the end, Oquendo said, many overdose deaths are classified as "undetermined."

One suicide expert explained why.

"If you're a coroner, it's not easy to discern intent," explained Jerry Reed, executive committee member of the National Action Alliance for Suicide Prevention, in Washington, D.C.

However, he said, it's known that both suicides and opioid overdose deaths have been rising.

According to the U.S. Centers for Disease Control and Prevention, the national suicide rate rose by 24 percent between 1999 and 2014 -- from 10.5 deaths per 100,000 people, to 13 per 100,000.

Meanwhile, the opioid toll keeps growing. Recent research has found a leveling off in Americans' abuse of prescription opioid painkillers -- like Vicodin, OxyContin and codeine. But abuse of illegal opioids, like heroin, is also rising.

And overall, opioid overdose deaths are still climbing.

Last year, a U.S government study highlighted the impact that heroin alone is having: Between 2002 and 2016, deaths from the drug soared by 533 percent nationwide -- from just under 2,100 deaths to more than 13,200.