The U.S. suicide rate is up for the first time in a decade, and it's rising most among middle-aged white men and women, a new study finds.

The researchers don't know why, but say we need to find out more so new prevention plans can be put in place.

Suicide claimed 32,637 lives in 2005, a rate of 11 per 100,000 people. The study, released this week, featured an analysis of data from 1999 to 2005. 

The rate increased 0.7 percent per year during that period. But when you break it down, the rate rose 2.7 percent annually among middle-aged white men and 3.9 percent among middle-aged white women. By contrast, suicide in blacks decreased significantly and remained stable among Asians and Native Americans, the researchers said.

Many people are surprised to learn that suicide is more common than homicide. However, suicides do not tend to rise during bad economic times.

Speculation for why the increase is occurring has ranged from increased drug use among Baby Boomers, who are known to be unhappy compared to other generations, to abuse of prescription drugs to changes among women in the use of hormone replacement therapy coincident with a 2002 report that found it potentially harmful. But the researchers say it is not clear if any of these factors are to blame.

"While it would be straightforward to attribute the results to a rise in so-called mid-life crises, recent studies find that middle age is mostly a time of relative security and emotional well-being," said Susan P. Baker of the Johns Hopkins Bloomberg School of Public Health. "Further research is warranted to explore societal changes that may be disproportionably affecting the middle-aged in this country."

Among white men and women, suicide has historically been less common in middle age. But by 2005, the 45-49 age group, for both sexes, had a higher rate than those age 40 and those age 70-74.

"Historically, suicide prevention programs have focused on groups considered to be at highest risk — teens and young adults of both genders as well as elderly white men, Baker said. "This research tells us we need to refocus our resources to develop prevention programs for men and women in their middle years."

Suicide by hanging or suffocation increased 6.3 percent a year among men and 2.3 percent among women. Hanging/suffocation accounted for 22 percent of all suicides by 2005, surpassing poisoning at 18 percent. Firearms represent the main method — about double the rate of any other — but have been on the decline.

The findings are detailed online in the American Journal of Preventive Medicine. The research was funded by the Center for Injury Research and Policy.