Radical cleric Anwar al-Awlaki was killed in a drone strike in Yemen in late September. (Associated Press)

Sen. Ron Wyden (Oregon), who has repeatedly accused the administration of excessive secrecy in national security matters, wrote to Attorney General Eric Holder in protest on Wednesday, saying it was “unacceptable” that the Justice Department had not been more forthcoming.

The killing of Awlaki, a prominent propagandist for al-Qaeda’s affiliate in Yemen, stoked a fierce debate over whether the Obama administration’s action was was lawful.

In 2010, Awlaki was placed on “kill lists” compiled by the CIA and the military’s Joint Special Operations Command. The Justice Department’s Office of Legal Counsel that year also completed a memo outlining why his killing would be lawful.

But to date, the administration has resisted disclosing the memo, despite repeated requests from civil liberties groups and others. Wyden said he asked Holder for details about the legal rationale last April. In May, Justice Department provided some information. But nine months later, he said, he is still waiting for the legal analysis.

“For the executive branch to claim that intelligence agencies have the authority to knowingly kill American citizens (subject to publicly unspecified limitations) while at the same time refusing to provide Congress with any and all legal opinions that delineate the executive branch’s understanding of this authority represents an indefensible assertion of executive prerogative, and I expected better from the Obama administration,” Wyden wrote in a letter.

The senator said he was not implying that the president lacked authority to approve the killing of an American citizen. But when the United States is fighting a terrorist group whose members do not wear uniforms but attempt to blend in with civilians, “questions about when the president may use lethal force against Americans whom he believes are part of this enemy force become significantly more complicated,” he wrote.

The Justice Department did not respond to immediate requests for comment.

Wyden has previously pressed the administration to release the legal rationale for an unnamed intelligence-gathering program authorized under Section 215 of the USA Patriot Act, a law passed after the 2001 terrorist attacks.