Islamist militias in Libya took control of nearly a dozen commercial jetliners last month, and western intelligence agencies recently issued a warning that the jets could be used in terrorist attacks across North Africa. Intelligence reports of the stolen jetliners were distributed within the U.S. government over the past two weeks and included a warning that one or more of the aircraft could be used in an attack later this month on the date marking the anniversary of the Sept. 11, 2001, terrorist attacks against New York and Washington, said U.S. officials familiar with the reports. “There are a number...