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WASHINGTON (AP) — An Air Force airman has been taken into custody in connection with an April explosion that injured several U.S. troops at a base in eastern Syria.

Rose Riley, an Air Force spokeswoman, said the airman was placed in pretrial confinement by his commander after a review of the investigation. An initial hearing on the matter is expected to take place Wednesday.

Riley said no charges have yet been filed, and the airman’s identity will not be released unless he is charged. The military has provided no details on the investigation.

The U.S. military initially reported that the injuries were caused by artillery or another form of indirect fire. But it later said the April 7 attack was carried out by the “deliberate placement of explosive charges” by one or more individuals at an ammunition holding area and shower facility on the base, known as Green Village.

The blasts hit two support buildings and four service members were evaluated for minor injuries and possible traumatic brain injuries.

The investigation is being done by the Army Criminal Investigation Division and the Air Force Office of Special Investigations.

The U.S. has several hundred troops in eastern Syria as part of its effort to counter the Islamic State group.