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The is investigating whether an attack at a base in eastern was an inside job, CNN reported on Monday.

"The investigation is ongoing, which may or may not, develop sufficient evidence to identify a perpetrator(s) and have enough evidence to ensure a conviction in a court of law," the Pentagon said in a statement to CNN, adding: "No further information will be released at this time."

Officials told the US news outletÌýthat the attack happened in the middle of the night, which suggests the perpetrator did not intend to cause mass casualties. The timing, however, suggests the suspect may have tried to escape quietly or quickly.

Four servicemen were injured in the attack.

The military’s Criminal Investigation Division and the Air Force Office of Special Investigations are conducting the investigation, CNN said.

In April, the US military said explosions earlier that month on the base known as Green Village were not, as it originally reported, caused by artillery or another form of indirect fire.

Instead, it is believed the 7 April 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.

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