WASHINGTON – Al-Qaida fighters have been using secretive chat rooms and encrypted Internet message boards for planning and coordinating attacks — including the threatened if vague plot that U.S. officials say closed 19 diplomatic posts across Africa and the Middle East for more than a week. It’s highly unlikely that al-Qaida’s top leader, Ayman al-Zawahri, or his chief lieutenant in Yemen, Nasser al-Wahishi, were personally part of the Internet chatter or, given the intense manhunt for both by U.S. spy agencies, that they ever go online or pick up the phone to discuss terrorist plots, experts say. But the unspecified...