Who Leaked The Assassination Memo?

Shafer wonders:

Washington often leaks in directions to further stoke policy fires that are already burning. (See this taxonomy of leaks compiled by Stephen Hess.) Such a Washington fire has been burning for many months, with Congress demanding that the Obama administration explain its targeted killings of U.S. citizens. Yesterday, before the Isikoff story broke, 11 senators sent the president a formal request that any and all legal opinions devised by the White House about the targeting of citizens be forwarded to the Judiciary and Intelligence Committees.

This request makes the senators, or members of their staffs, prime suspects of the leak.

Reax to the memo here. Mine is forthcoming.