In fact, other top officials of the group were the strike’s specific targets, and Mr. Awlaki’s death would have been collateral damage — legally defensible as a death incidental to the military aim. As dangerous as Mr. Awlaki seemed, he was proved to be only an inciter; counterterrorism analysts did not yet have incontrovertible evidence that he was, in their language, “operational.”

That would soon change. The next day, a 23-year-old Nigerian named Umar Farouk Abdulmutallab tried and failed to blow up an airliner as it approached Detroit. The would-be underwear bomber told F.B.I. agents that after he went to Yemen and tracked down Mr. Awlaki, his online hero, the cleric had discussed “martyrdom and jihad” with him, approved him for a suicide mission, helped him prepare a martyrdom video and directed him to detonate his bomb over United States territory, according to court documents.

In his initial 50-minute interrogation on Dec. 25, 2009, before he stopped speaking for a month, Mr. Abdulmutallab said he had been sent by a terrorist named Abu Tarek, although intelligence agencies quickly found indications that Mr. Awlaki was probably involved. When Mr. Abdulmutallab resumed cooperating with interrogators in late January, an official said, he admitted that “Abu Tarek” was Mr. Awlaki. With the Nigerian’s statements, American officials had witness confirmation that Mr. Awlaki was clearly a direct plotter, no longer just a dangerous propagandist.

“He had been on the radar all along, but it was Abdulmutallab’s testimony that really sealed it in my mind that this guy was dangerous and that we needed to go after him,” said Dennis C. Blair, then director of national intelligence.

A Legal Quandary

David Barron and Martin Lederman had a problem. As lawyers in the Justice Department’s Office of Legal Counsel, it had fallen to them to declare whether deliberately killing Mr. Awlaki, despite his citizenship, would be lawful, assuming it was not feasible to capture him. The question raised a complex tangle of potential obstacles under both international and domestic law, and Mr. Awlaki might be located at any moment.

According to officials familiar with the deliberations, the lawyers threw themselves into the project and swiftly completed a short memorandum. It preliminarily concluded, based on the evidence available at the time, that Mr. Awlaki was a lawful target because he was participating in the war with Al Qaeda and also because he was a specific threat to the country. The overlapping reasoning justified a strike either by the Pentagon, which generally operated within the Congressional authorization to use military force against Al Qaeda, or by the C.I.A., a civilian agency which generally operated within a “national self-defense” framework deriving from a president’s security powers.