Four U.S. troops died in an attack on Bagram Air Base in Afghanistan on Tuesday.

According to an Associated Press report, the soldiers were killed by “indirect fire,” such as a mortar or rocket. The report cites anonymous officials who could provide no more detail on the attack or deaths.

Bagram Air Base was originally an airport, built during the 1950s. The site was used by the U.S.S.R. during their invasion of Afghanistan in the 80s. After the war, the airport was a heavily contested site between Northern Alliance and Taliban forces. With the U.S.-led invasion of Afghanistan in 2001, the site was quickly secured for the use of NATO troops. The airport’s runways have since been repaired, and even more runways have been constructed for the use of larger aircraft.

According to the AP, the attack came just as U.S. troops were handing over control of Afghanistan security to the Afghan army and police. U.S. troops will not, however, be fully withdrawing from the country just yet. They will be staying on to consult with Afghan troops, as well as to provide back-up, training, and air support.

(via AP)