Two top senators served the Obama administration Monday with subpoenas for information on the mass shooting at Fort Hood last November, claiming the administration's stonewalling left them with no other choice. 

Sen. Joe Lieberman, I-Conn., and Sen. Susan Collins, R-Maine -- respectively, the chairman and the ranking Republican on the Senate homeland security committee -- notified Attorney General Eric Holder and Defense Secretary Robert Gates of the decision in a letter Monday. 

They said the committee had sent four formal requests for information to the Pentagon and two to the Justice Department, and received little response. 

"Our efforts have been met with delay, the production of little that was not already publicly available, and shifting reasons why the departments are withholding the documents and witnesses that we have requested," they wrote. "Unfortunately, it is impossible for us to avoid reaching the conclusion that the departments simply do not want to cooperate with our investigation."

They said the subpoenas were being issued "with great disappointment and reluctance." 

As part of their investigation, Lieberman and Collins are calling on the administration to release prosecutorial interviews with witnesses, a supplement to the Pentagon's independent review, and the personnel files for suspect Maj. Nidal Hasan. They also want to have interviews arranged with Pentagon and FBI officials who investigated Hasan after his contact with radical cleric Anwar al-Awlaki. 

They called for the materials to be released by next Monday. 

Lawmakers have long complained that the administration is holding back on releasing details surrounding the shooting.

But the Defense and Justice departments say that release of the disputed data would compromise the prosecution of Hasan, who is charged with killing 13 people. 

"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.

The Associated Press contributed to this report.