Ron Wyden is question the legality of Intelligence tracking smartphones. Reuters

Sen. Ron Wyden, D-Ore., has joined a chorus of critics demanding that the Obama administration release its legal justification for assassinating the radical Muslim cleric Anwar al-Awlaki, a U.S. citizen, in Yemen.

President Barack Obama has subscribed to a muscular interpretation of executive power, adopting and at times expanding upon the authority to kill or indefinitely detain terrorists first claimed by President George W. Bush. That included Obama's authorization of a September 2011 drone strike in Yemen that killed Awlaki, who had never been formally charged with a crime.

The Justice Department has steadfastly refused to release legal opinions explaining why Obama had the authority to launch the Awlaki strike, drawing lawsuits from the American Civil Liberties Union and the New York Times. Wyden sent a letter to Attorney General Eric Holder on Wednesday saying that the Justice Department's silence represented an indefensible assertion of executive prerogative.

Wyden did not challenge the legality of Obama's actions, writing that the president should have extraordinary powers in a time of war. But he wrote that questions about when the president should be allowed to deploy lethal force become far more complicated when fighting terrorists who do not wear uniforms or join conventional armies.

The federal government's official views about the president's authority to kill specific Americans who have not necessarily been convicted of a crime are not a matter to be settled in secret by a small number of government lawyers, Wyden wrote. Instead, the government's interpretation of relevant statutory and constitutional protections should be public knowledge, so that they can be publicly debated and understood.

Wyden added that sweeping new powers previously claimed by Bush did not stand up to legal scrutiny.

The only way to keep the terrible mistakes of the past from being repeated is to have executive branch officials continually resist the temptation to rely on secret law, and for Congress and the public to continually insist on greater transparency, Wyden wrote.

Wyden has been a leading critic of government secrecy. During a May debate over renewing the Patriot Act, he warned that the federal government was asserting vast surveillance and information powers that would leave most Americans stunned.