A Senate committee on Monday subpoenaed the Obama administration for secret documents and access to witnesses in last year's mass shooting at the Fort Hood Army base in Texas.
The subpoena is a rare public dispute between a Democratic-led Congress, which has been largely supportive of President Barack Obama's policies, and the new administration, which prides itself on increased government transparency.
Lawmakers have alleged that the administration is covering up critical details on the case, including whether the government had access to information that could have prevented the shooting.
"Unfortunately, it is impossible for us to avoid reaching the conclusion that the departments simply do not want to cooperate with our investigation," wrote Sens. Joseph Lieberman and Susan Collins in a letter accompanying the subpoena.
Lieberman, a Connecticut Independent, chairs the Senate Homeland Security and Governmental Affairs Committee; Collins, a moderate, is the panel's top Republican.
"We'll obviously be reviewing it and determining the department's next steps," Pentagon spokesman Bryan Whitman said of the subpoena.
Some of the dispute boils down to technicalities. For example, the Defense Department has already given temporary access of some personnel files to another congressional committee.
The case of Hasan has been of particular interest to lawmakers because of his suspected ties to foreign terrorists and his promotion through the military's ranks despite repeated concerns over his performance and behavior.
An internal Defense Department review concluded that several unidentified medical officers failed to use "appropriate judgment and standards of officership" when reviewing Hasan's performance as a student, internist and psychiatric resident.
Last week, Gates said the Pentagon would provide Congress with any information it could so long as the information wouldn't hurt the prosecution's case.
"We have no interest in hiding anything, but what's most important is that prosecution," he said.
Gates also has issued new regulations, including restrictions on how privately owned guns can be carried or stored at military installations. Hasan had little or no access to military firearms in his job as a psychologist, but was able to buy two handguns and bring them onto the base.