Here is a link to the WaPo story regarding Senators Lieberman and Collins “first congressional subpoenas of the Obama administration Monday for witnesses and documents about the Fort Hood shootings.”
According to The Hill, here, DoD responded saying,
We will continue to cooperate with the committee in every way, with that single caveat that whatever we provide does not impact our ability to prosecute . . . The Department has and continues to cooperate with Congress while ensuring the integrity of both our own investigation as well as the criminal investigation and prosecution of Nidal Hasan.