The three Army Special Forces soldiers killed at a Jordanian military base this month were working for a CIA program to train moderate Syrian fighters when they were shot at a checkpoint under still-unclear circumstances, U.S. officials said. The Nov. 4 slayings of the three soldiers is believed to be the deadliest single incident involving a CIA team since December 2009, when seven officers and contractors were killed in a suicide bombing in Khost, Afghanistan. The Fort Campbell, Ky.-based soldiers — all members of the 5th Special Forces Group — were killed by a Jordanian soldier at an entry control...