Through it all, Yoo came down on the side of interpreting the Constitution to give the president extensive latitude in protecting American interests from military threat. After the Sept. 11 terror attacks, he said, an atmosphere of urgency prevailed because the threat was so great.
"The government sometimes . . . has to ask young men and women to kill other people," he said. "Our job [in the Justice Department] is to say what the law allows and doesn't allow."
Yoo is best known as the author of memos - while a Justice Department lawyer - that provided the legal basis for harsh interrogation techniques employed by the CIA on al-Qaeda detainees at Guantanamo and other facilities. He played a central role in shaping the Bush administration's handling of captured al-Qaeda prisoners, arguing that the Geneva Conventions affording protections to enemy soldiers did not apply to members of terrorist organizations that were not signatories to the agreement.
Yoo and other Justice Department lawyers concluded that only interrogation techniques inflicting severe pain or severe and permanent bodily injury, such as organ damage, and death were banned from use by U.S. forces.
The administration argued that such techniques, including wall-slamming and waterboarding - the near-drowning of a person - were necessary to elicit critical information to protect the country from attack.
Yoo, who was raised in Philadelphia and attended Episcopal Academy, was born in Seoul, South Korea. He attended Harvard University as an undergraduate and earned his law degree at Yale University. He is a law professor at the University of California at Berkeley and is a visiting scholar at the conservative American Enterprise Institute in Washington. He also is a former Inquirer columnist.
In 2010, little more than a year after President Obama had taken office, the Justice Department's Office of Professional Responsibility concluded that Yoo and another colleague at Justice had committed "professional misconduct" in drafting the legal opinions and recommended that the matter be referred to bar regulators for possible sanctions.
But the office was overruled by more senior Justice Department officials, who concluded that although Yoo and his colleague Jay S. Bybee had provided flawed legal advice, it did not rise to the level of misconduct.
In earlier comments on the issue, Yoo dismissed the Office of Professional Responsibility report as having been influenced by public opinion, and he challenged his critics to say what they would do to protect the country in the face of an imminent attack, and the need for intelligence.
On the NSA collection of data on millions of phone calls by American citizens, Yoo said the program likely was constitutional, and justified under a legal doctrine holding that citizens give up their right to keep such information private once they turn it over to a third party, such as a phone company.
The greater risk, he says, is what the government does with that information once it has it in hand.
"What should bother people is not the collection but rather the analysis," Yoo said. "There is a torrent of information available on people now."