The Obama administration secretly gave its blessing to arms shipments to Libyan rebels from Qatar last year, but American officials later grew alarmed as evidence grew that Qatar was turning some of the weapons over to Islamic militants, according to United States officials and foreign diplomats. No evidence has emerged linking the weapons provided by the Qataris during the uprising against Col. Muammar el-Qaddafi to the attack that killed four Americans at the United States diplomatic compound in Benghazi, Libya, in September. But in the months before, the Obama administration clearly was worried about the consequences of its hidden hand...