The American Civil Liberties Union on Wednesday filed a lawsuit against the Department of Justice, Department of Defense and CIA in hopes of disclosing government information related to the killing of three U.S. citizens by drone attacks last year.

"Our government’s deliberate and premeditated killing of American terrorism suspects raises profound questions that ought to be the subject of public debate," Nathan Freed Wessler of the ACLU said. "Unfortunately the Obama administration has released very little information about the practice — its official position is that the targeted killing program is a state secret — and some of the information it has released has been misleading."

Airborne drones operated by the U.S. reportedly killed alleged al Qaeda operatives Anwar al-Awlaki and Samir Kahn last year in Yemen. Both were U.S. citizens.

President Obama described the killing of Awlaki as a "significant milestone" in the fight against al Qaeda. But did not say whether he was killed collaterally or was intentionally targeted.

Awlaki's 16-year-old son, Abdulrahman, was reportedly killed by a drone strike about two weeks later. He was also a U.S. citizen.

More than a year before the death of Awlaki, the media reported that he had been placed on a "hit list" by the CIA and Joint Special Operations Command after intelligence agencies concluded the cleric was directly involved in terror plots.

A Department of Justice document, the OLC memo, reportedly provided the legal justification for the killing of a U.S. citizen without due process. The memo has not been released to the public.

The ACLU filed a Freedom of Information Act (FOIA) request in hopes of obtaining the memo and other documents relating to the "hit list." But the government declined to release anything.

"The public has a right to know the evidence and legal basis for the deliberate targeted killing of U.S. citizens," Wessler said. "So chilling a power must be opened to public scrutiny and debate."