On Dec. 25, 2009, a young Nigerian equipped by Mr. Asiri with explosives hidden in his underwear, Umar Farouk Abdulmutallab, made it through airline security and onto a flight from Amsterdam to Detroit. But when he tried to blow up the flight as it approached Detroit, the bomb only ignited and burned Mr. Abdulmutallab, who was swiftly subdued by other passengers.

Some intelligence officials expressed concern after that failed attack that Mr. Asiri had recruited one or more surgeons to experiment with implanting a bomb with no metal parts into the abdomen of a suicide bomber. There is no known case in which such an attack was carried out, but the British documents suggest that intelligence officials believed Dr. Usamah was part of an effort to develop such plans when he was killed.

Some of the British agency’s documents suggest, though they do not explicitly state, that it provided intelligence for that strike in Yemen and other American strikes. That would be no surprise, since intelligence cooperation between the United States and Britain has long been close, particularly in the area of signals intelligence, or eavesdropping. The documents discuss the British agency’s employees who work at an N.S.A. station in Fort Gordon, Ga., and at a large N.S.A. center in England called Menwith Hill station.

British officials rarely speak publicly about cooperation with the program of targeted killings. In a formal answer to a parliamentary inquiry last year about whether Britain was participating in unmanned aerial vehicle strikes in Yemen, the British defense minister, Mark Francois, replied in writing that “U.A.V. strikes against terrorist targets in Yemen are a matter for the Yemeni and U.S. governments.” The answer did not explicitly deny a British role, but certainly suggested there was not one.

American drone strikes are supported by a majority of the public in the United States but opposed by the British public. A poll last year by the Pew Research Center found that 52 percent of Americans backed the strikes, with 41 percent opposed. In Britain, 59 percent were opposed, while 33 percent approved of the strikes.

Opposition to the strikes is often based on reports of civilians killed unintentionally. While proponents argue that missiles fired from unmanned aircraft are the most precise way to eliminate terrorists, intelligence agencies often do not have enough detailed information about who is in a strike zone to be certain that all are militants posing a threat to the United States or to Americans overseas.