Why won't the Obama administration answer questions about the Fort Hood Massacre?

From the NY Daily News:

Two top senators on Thursday accused the Obama administration of stonewalling their probe of the Fort Hood slaughter.

Senate Homeland Security Committee chairman Joseph Lieberman (I-Conn.) and Sen. Susan Collins (R-Maine) said they'll subpoena the Justice Department and Pentagon by Monday if documents aren't coughed up fast.

The lawmakers want to know who knew Army Maj. Nidal Hasan was chatting up a radical Al Qaeda cleric, and why they didn't tell Hasan's superiors.

"The idea [our probe] somehow is going to compromise the ability to prosecute the case is just foolish," Collins complained. . . .