Some lawmakers believe the Obama administration may be covering up information about the Fort Hood shooting.
A Senate committee issued subpoenas Tuesday, for details regarding the Nov. 5 attack that left 13 people dead and whether it could have been prevented.
"Unfortunately, it is impossible for us to avoid reaching the conclusion that the departments simply do not want to cooperate with our investigation," Sens. Joseph Lieberman and Susan Collins wrote in a letter accompanying the subpoena.
The Justice Department says releasing documents and allowing access to witnesses could compromise the prosecution of accused gunman Nidal Hasan.
"We have no interest in hiding anything, but what's most important is that prosecution," Defense Secretary Robert Gates said.
Lawmakers want to know why Hasan was promoted through military ranks despite his suspected ties to terrorists.