(CNN) Three men were taken into custody Wednesday after a vehicle attempted to enter the National Security Agency's headquarters in Fort Meade, Maryland, the FBI said.

The driver of the car, as well as a NSA police officer and a civilian onlooker, were injured in the incident, FBI Special Agent in Charge Gordon Johnson said at an afternoon news conference. Gunshots were fired but the FBI doesn't believe anyone was shot.

Two of the individuals are in NSA custody and being interviewed by FBI agents as part of the bureau's investigation, Johnson said. The third -- the driver of the vehicle -- is being treated in the hospital for his injuries. The NSA police officer and civilian were also taken to the hospital for non-life threatening injuries.

Investigators believe that the incident, which happened shortly before 7 a.m. ET, is isolated and not terror-related, Johnson said.

Johnson said the FBI is looking into why gunshots were fired, saying it is part of the investigation. Another central question is why the vehicle came onto the compound, he said.

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