An Afghan soldier opened fire at a military airport in Kabul on Thursday, killing three American contract workers and wounding a fourth, according to Reuters, which labelled the shooting an “insider attack.”

"It is unclear yet why he shot these advisers and no one else was there to tell us the reason," an Afghan air force official told Reuters, speaking on condition of anonymity. "An investigation has been opened."

Kabul’s military airport is a major hub for international travel, particularly for American contractors and aid workers. The Americans killed on Thursday were “trainers,” according to the Afghan official who spoke to Reuters. There are more than 1,000 Departement of Defense trainers still in Afghanistan, whose responsibility is to provide instruction to Afghanistan’s security forces but do not engage in combat, according to the January census report by the U.S. Central Command (CENTCOM.)

There are approximately 39, 609 contractors stationed in Afghanistan working with the U.S. Department of Defense, 14,222 of whom are American citizens. In addition, the DoD has roughly 1,511 private security contractors in the country who “perform personal security, convoy security, and static security missions,” according to CENTCOM.

Earlier this week, the United States Agency for International Development (USAID) announced that it suspended the International Relief and Development, one of its largest nonprofit contracting companies in Afghanistan.  A USAID investigation found “serious misconduct” including mischarging millions of dollars in overhead costs, according to the Washington Post.

The American combat mission in Afghanistan officially ended in December. The U.S. has since started the slow process of pulling out of the country, meaning Afghanistan’s armed forces are the only body fighting what the White House recently called the Taliban’s “armed insurgency” gripping the country.  

Though the combat mission just recently ended, Afghan security forces assumed full responsibility from foreign forces in 2013. Since then, they have faced an alarmingly high death toll. At least 4,634 Afghan security forces were killed in 2014 and 4,350 died the year before, Lt. Gen. Joseph Anderson, a senior commander for U.S. and NATO forces in Afghanistan, told the Washington Post in November.