LONDON — Prime Minister David Cameron said Monday that a British aid worker killed in an American rescue raid in Afghanistan last week may have been killed by a grenade detonated by a United States special forces unit — not in an explosion of a suicide bomber’s vest detonated by her Taliban captors, as the American command in Afghanistan suggested when it confirmed her death on Saturday. A grim-faced Mr. Cameron appeared at a news conference at 10 Downing Street to say he had learned of “this deeply distressing development” Monday from the top American and NATO commander in Afghanistan,...