U.S. intelligence officials appear to have obtained access to what could turn out to be a significant trove of phone numbers, photographs and documents detailing the links between Al Qaeda's leaders in northwest Pakistan and the terror group's increasingly menacing affiliate in Yemen, two counter-terrorism sources tell Declassified. In late January, an Al Qaeda operative headed from Pakistan on his way to Yemen was arrested in the Persian Gulf country of Oman, a U.S. counter-terrorism official confirmed. There has been no public announcement of the arrest. But in a possible indication of the operative's importance, just a few days later,...