The man in charge of America's drone wars will face Senate questioning about perhaps their most controversial aspect: when the president can target American citizens for death.

Sen. Ron Wyden (D-Ore.) sent a letter on Monday to John Brennan, the White House's counterterrorism adviser and nominee to be head of the CIA, asking for an outline of the legal and practical rules that underpin the U.S. government's targeted killing of American citizens suspected of working with al-Qaida. The Obama administration has repeatedly resisted disclosing any such information about its so-called "disposition matrix" targeting terrorists, especially where it concerns possible American targets. Brennan reportedly oversees that matrix from his White House perch, and would be responsible for its execution at CIA director.

"How much evidence does the President need to determine that a particular American can be lawfully killed?" Wyden, a member of the Senate intelligence committee, asks in the letter, acquired by Danger Room. "Does the President have to provide individual Americans with the opportunity to surrender before killing them?"

Wyden's questions about the targeted-killing effort get specific. He wants to know how the administration determines when it is "not feasible" to capture American citizens suspected of terrorism; if the administration considers its authority to order such killings inherent in its Constitutional war powers or embedded in the 2001 Authorization to Use Military Force; and if the intelligence agencies can "carry out lethal operations inside the United States." Wyden also expresses "surprise and dismay" that the intelligence agencies haven't provided him with a complete list of countries in which they've killed people in the war on terrorism, which he says "reflects poorly on the Obama administration's commitment to cooperation with congressional oversight."

Thus far, senators on the intelligence panel have been more concerned about Brennan's possible role in national-security information leaks and the CIA's post-9/11 torture program than in using Brennan's nomination to peer into the decision-making surrounding Obama's counterterrorism strikes. Wyden writes that it is "critically important" for Congress to understand "how the executive branch understands the limits and boundaries of this authority."

In September 2011, a U.S. missile strike in Yemen killed Anwar al-Awlaki, al-Qaida's most prominent English-language propagandist and an American citizen. Weeks later, another strike fired by a U.S. drone killed Awlaki's 16-year old son, Abdulrahman.

The Obama administration has never disclosed the evidence behind its claims that the elder Awlaki posed such an imminent danger to Americans that prompted killing him without due process of law, prompting a major debate about the legality of the killing. (Administration officials have said even less about the justification for killing the 16-year old Abdulrahman al-Awlaki.) Members of the Awlaki family, the New York Times and the American Civil Liberties Union have variously sued the government for additional information about the strikes, all unsuccessfully. Earlier this month, a federal judge in New York ruled that the government was not required to disclose the legal analysis undergirding the Awlaki targeting decisions, even as the judge herself blasted the administration for embracing "certain actions that seem on their face incompatible with our Constitution and laws, while keeping the reasons for their conclusion a secret."

Wyden doesn't specifically ask for information about the killing of Awlaki and his son. He's after the more general rules embraced by the administration about when killing a U.S. citizen is permissible in the U.S. shadow wars. Those rules, apparently written in 2010 by Justice Department lawyers David Barron and Marty Lederman, were described in a New York Times piece but remain secret. Wyden doesn't ask for their disclosure, just for the administration to permit members of the intelligence panel to read them.

"For the executive branch to claim that intelligence agencies have the authority to knowingly kill American citizens but refuse to provide Congress with any and all legal opinions that explain the executive branch's understanding of this authority represents an alarming and indefensible assertion of executive prerogative," Wyden writes.

As the Senate intelligence panel's leading civil libertarian, Wyden frequently calls on the administration to explain the classified reasoning behind its most controversial national security practices. He's accused the administration of adopting a private interpretation of the Patriot Act so expansive, for instance, that it amounts to "secret law" authorizing surveillance. Most often the administration stiffs Wyden. The National Security Agency has yet to disclose how many Americans are caught up in its dragnet for terrorist communications, one of Wyden's major preoccupations, telling the Senator that even disclosing that number would violate Americans' privacy.

Brennan remains likely to receive Senate approval for his CIA nomination. But until now, senators on the panel that will handle his confirmation had seemed less than interested in exploring the more discomforting aspects of the counterterrorism strategy colloquially referred to as the "drone war."

Wyden doesn't endorse or reject Brennan for the top CIA job. Wyden spokesman Tom Caiazza tells Danger Room that "the senator is looking forward to a frank and substantive conversation on these issues and of course the results will be considered in his decision making."