Sen. Mark Udall, D-Colo.
Sen. Mark Udall, D-Colo. (AP | Susan Walsh)

WASHINGTON — Sen. Mark Udall said he was frustrated Thursday that he did not get substantive time — and staff expertise — with classified documents that lay out a legal rationale for drone strikes against Americans working with al-Qaeda abroad.

President Barack Obama released a confidential legal opinion to senators on the Intelligence Committee on Thursday — in part because Udall, a Colorado Democrat, and Sen Ron Wyden of Oregon pushed aggressively for transparency on the government's constitutional justification to kill American-born citizens abroad in counterterrorism operations.

But the senators on Thursday were not allowed to bring in staffers or their own lawyers to help them parse the legal language, Udall said. He said he had only 15 to 20 minutes with the complex legal opinion early in the morning because his day was so packed with other obligations.

"The restrictions on what I can say or what I can't say are frustrating," Udall said. "The point of the memos as a committee is how the president is justifying the killing of Americans on foreign soil. ... The legal document we saw is detailed and substantive enough, but even if you're a lawyer ... it's extremely complicated."

Given that, Udall said he thinks there "could be a strong case" for the United States to continue the targeted killing program, but he wants more time, and some legal help, before deciding for sure.

Obama called Udall late Wednesday and urged him to support his pick, John Brennan, to run the CIA. Obama told Udall that he would let some senators read the legal justification of the nation's targeted killing program Thursday morning, ahead of Brennan's confirmation hearing.

Udall said he told Obama that he respected his opinion but that he wanted to understand the Justice Department's rationale before making up his mind on Brennan.

"That need wasn't made out of spite or disagreement of his (Brennan's) record," Udall said. "But this is a point that's really important."

This isn't the first time Udall has pushed the executive branch on transparency and constitutional liberties. Two years ago, Udall urged reform of the Patriot Act so the government couldn't have broad authority to tap Americans' cellphones.

"The American people have the right to know what the government does on their behalf," Udall said at Brennan's confirmation hearing Thursday. "The presumption of transparency should be the rule, not the exception. The government should make as much information available to the American public as possible."

Brennan's confirmation hearing comes the same week as the leak of Justice Department documents justifying the reason an American citizen was killed in a drone attack in Yemen in 2011.

Udall said he had seen those previously classified memos but sought a deeper legal understanding of how the United States constitutionally justifies killing American citizens without due process.

Americans are afforded constitutional privileges both on the homeland and abroad — making the killing program a complex legal tangle between civil liberties and the executive and legislative branches of government.

"It seems to me there is no good reason why the administration shouldn't be able to disclose more about the internal decision-making process, what kind of committee they have, who makes the final decision," said Daniel Marcus, a law professor at American University's Washington College of Law and former general counselor for the 911 Commission. "I'm sympathetic to the need for some secrecy, but even there, you'd think they could provide a summary."

Allison Sherry: 202-662-8907, or