According to an NPR report repeated at the Stars and Stripes, the FBI has investigated more than 100 suspected Islamic extremists “within the US military community:”

About a dozen of the cases were considered serious, according to the report…

The suspects were those who allegedly demonstrated a strong intent to attack military targets…
 
Officials said the suspects could be active and reserve military personnel or individuals who have military facilities access, such as contractors and family members with dependent ID cards, NPR reported.

The investigations may well be in response to criticisms that the military was negligent in failing to see the (alleged) threat in US Army Major Nidal Malik Hasan.

Likewise, of 12 attempted or successful attacks against the US military that were apparently motivated by the attacker’s Islamic faith, four were committed by uniformed members of the US military.