The U.S. military on Monday announced that a fellow service member may have been responsible for an attack on April 7 at a small military base in Syria that wounded four troops. The attack was initially believed to have come from indirect fire.

CNN first reported the investigation by the U.S. Army Criminal Investigation Division and the Air Force Office of Special Investigations, citing three defense officials and a statement from the military.

The military had concluded a week after the incident that there was a "deliberate placement of explosive charges by an unidentified individual(s) at an ammunition holding area and shower facility."

The unidentified suspect is believed to no longer be in Syria, which has about 900 U.S. military troops stationed in the northeast part of the country. The military has maintained a presence in the region to suppress cells of Islamic State insurgents. Iran-backed forces and Syrian military units are also known to operate in this area. 

The four injured service members suffered potential traumatic brain injuries.

Inside attacks were an ever-present danger for U.S. forces during the war in Afghanistan. In a number of incidents, Afghan soldiers, sometimes disguised members of the Taliban, opened fire on U.S. and pro-U.S. Afghan forces. 

Attacks by fellow troops have been rare. The last confirmed instance of service members turning on each other in a combat zone was in 2005 when an Army soldier was accused of killing his company commanders with explosives. The soldier was ultimately acquitted of the charge.