The US military says explosions earlier this month on a base in eastern Syria that injured several US service members were not, as it originally reported, caused by artillery or another form of indirect fire.

Instead, it is now believed 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, according to a statement issued Thursday by the Operation Inherent Resolve command that oversees US military operations against the ISIS group in Syria and Iraq.

The statement said the incident is under investigation but provided no other details.

The reference to “deliberate placement” of explosive charges on the base appeared to raise the prospect of infiltration and a lapse of base security.

In its original announcement on April 7, the command said the base, known as Green Village, received two rounds of indirect fire that struck two support buildings. Four service members were being evaluated for minor injuries and possible traumatic brain injuries.

The US has several hundred troops in eastern Syria as part of its effort to counter the ISIS group.