The reasons for the Air Force increase are not fully understood, coming after years of effort by all of the military services to counter a problem that seems to defy solution and that parallels increases in suicide in the U.S. civilian population.

According to preliminary figures, the Air Force had 84 suicides among active-duty members last year, up from 60 the year before. The jump followed five years of relative stability, with the service's yearly totals fluctuating between 60 and 64. Official figures won't be published until later this year and could vary slightly from preliminary data.

Air Force officials, who confirmed the 2019 total, said they knew of no higher number in recent years. Data and studies previously published by the Pentagon and Air Force show that 64 suicides in 2015 had been the highest total for the Air Force in this century. A 2009 Air Force study said suicides between 1990 and 2004 averaged 42 a year and never exceeded 62.

"Suicide is a difficult national problem without easily identifiable solutions that has the full attention of leadership," Lt. Gen. Brian Kelly, the Air Force's deputy chief of staff for manpower, personnel and services, said in a statement. He said the Air Force is focused on immediate, midterm and long-range solutions to a problem faced throughout the military.

Suicide risk factors are often thought to include stress related to deployment to combat zones in Iraq and Afghanistan. But a study published in the Journal of the American Medical Association in 2013 concluded, based on an assessment of current and former military personnel over a seven-year period, that combat experience and other deployment-related factors were not associated with increased risk of suicide. Instead the study's results pointed to numerous other factors, including being male, engaging in heavy or binge drinking, and bipolar disorder.

Although only the Air Force saw a major increase last year, all the services have struggled with higher suicides since about 2005-2006, which coincided with a cycle of exceptionally stressful deployments to Iraq for the Army and Marine Corps. The Pentagon encourages service members and veterans in need of help to contact the Military Crisis Line.

The Navy last year saw its active-duty suicides rise by four, to 72, and the Marine Corps total dropped by 10, to 47. All the 2019 numbers include confirmed and suspected suicides and are subject to revision based on further medical review. It is not uncommon for a service's total to get adjusted up or down after further review, but any changes are slight.

The Army declined to reveal its 2019 preliminary total, but The Associated Press determined it was little changed from the previous year's 139. The Army's figure is typically the highest in the military because it is by far the biggest service, with about 480,000 soldiers on active duty this year, compared with about 332,000 in the Air Force.

The Air Force in the mid-1990s pioneered a suicide prevention program that was seen as effective, and at various times since the U.S. became entangled in combat in Iraq and Afghanistan the other services have seen troubling increases in their suicide numbers. The Marine Corps, for example, saw its numbers jump from 37 to 57 between 2016 and 2018.

Maj. Craig W. Thomas, a Marine Corps spokesman, said the Marines want further progress after recording 10 fewer active-duty suicides last year. He said unit leaders are encouraged to speak openly with their Marines about stress, mental wellness and suicide.

"When leaders and mental health programs and resources acknowledge that 'everybody struggles with life, trauma, shame, guilt and uncertainty,' it helps make asking for assistance more acceptable," Thomas said.

Last year, the Air Force went public with its concerns as it saw its suicide numbers rising. Last summer, Gen. David Goldfein, the Air Force chief of staff, ordered a "resilience tactical pause" across the force to foster open discussion within the service about suicide prevention. In a July 31 letter, he wrote: "Hopeful to hopeless. What is going on? It is our job to find out."

Answers are elusive, but the Air Force says the Goldfein "pause" jump-started an effort to promote "connectedness" among airmen.

The military, whose population is generally younger and more fit than America as a whole, is quick to note that suicide is a problem throughout society. According to the Centers for Disease Control and Prevention, from 1999 through 2017, the nation's suicide rates increased for both men and women, with bigger percentage increases occurring after 2006.