The suicide rate in the United States rose 24 percent between 1999 and 2014, according to a new report by the Centers for Disease Control and Prevention (CDC). The researchers say increasing drug use may be one of the contributing factors.

The economy is another possible factor in the increasing suicide rate, The Wall Street Journal reports.

Suicide is the tenth-leading cause of death in the United States, the report notes. The suicide rate continued to increase in the first half of 2015, the CDC found in a separate study. There were more suicides among men than women, but the suicide rate for women increased faster during the study period.

“Exactly where the major influences are—we don’t know all of the answers to that yet,” said CDC researcher Alex Crosby. “It is usually an interaction of multiple behaviors.”

He noted abuse of prescription opioids, heroin and other drugs has increased in the past 15 years. According to the CDC, more people died from drug overdoses in 2014 than in any year on record. The majority of drug overdose deaths involve an opioid. Since 1999, the rate of overdose deaths involving opioids (including prescription opioid pain relievers and heroin) almost quadrupled. From 2000 to 2014 nearly half a million people died from drug overdoses. Those deaths include suicides, the article noted.

Crosby noted the economic downturn also may have contributed to the suicide rate, which historically has risen during difficult economic times. Lack of easy access to mental health services may also play a role in the rising rate, he added.

Suicides surged among middle-aged men and women, the report found. The suicide rate tripled among young girls, ages 5 to 14. The actual number of suicides in that age group remains small, the researchers said.